Improve find and package support

Whitelist of repos:
https://github.com/travis-ci/apt-source-whitelist/blob/master/ubuntu.json

Whitelist of packages:
https://github.com/travis-ci/apt-package-whitelist/blob/master/ubuntu-precise

The use of `CPACK_PACKAGE_NAME` leads to
`cppformat-<version>-Source.zip` as the name for the source package
which is different from the expected `cppformat-<version>.zip`.

Changed ArgMap to be backed by a vector instead of a map.

This commit upgrades cmake to 2.8.12 to implement proper cmake
`find_package` support using config and export file generation.
Having this support enables users to use installed cppformat
with a simple `find_package` call. Directly using a version
from a build directory is also supported.

main.cpp:
```
 #include <cppformat/format.h>
int main(int argc, char** argv)
{
  for(int i = 0; i < argc; ++i)
    fmt::print("{}: {}\n",i,argv[i]);
  return 0;
}

```

CMakeLists.txt:
```
cmake_minimum_required(VERSION 2.8.12)

project(cppformat-test)

find_package(cppformat REQUIRED)

add_executable(cppformat-test "main.cpp")
target_link_libraries(cppformat-test cppformat)

```
Configuring when cppformat is installed under `CMAKE_INSTALL_PREFIX`: `cmake <PATH_TO_TEST_SRC>`

Configuring when cppformat is installed `ELSEWHERE`: `cmake -Dcppformat_DIR=<ELSEWHERE>/lib/cmake/cppformat <PATH_TO_TEST_SRC>`

Configuring when cppformat is only built: `cmake -Dcppformat_DIR=<cppformat_BUILD_DIR> <PATH_TO_TEST_SRC>`

The main reason for this is to avoid a dynamic memory allocation in every format() call with Visual Studio if there are no named arguments.

Fixed unknown pragma warnings for _BitScanReverse.

This is mainly just to avoid including intrin.h unnecessarily.

Changed to only define the MSVC implementations for clz and clzll if the builtins are not available to avoid warnings about an unknown #pragma for "intrinsic".

Fixed macro redefinition warnings when compiling with clang-cl.

Both clang-cl and Clang/C2 #define _MSC_VER but also have support for __builtin_clz and __builtin_clzll, leading to duplicate macro definition warnings. Emulation of clz using _BitScanReverse is suppressed if the builtins are already available.

Additionally, the value of the output parameter of _BitScanReverse is undefined if the input value is 0, which is avoided by construction, so the code analysis warning for using uninitialized data is now suppressed.

Both clang-cl and Clang/C2 #define _MSC_VER but also have support for __builtin_clz and __builtin_clzll, leading to duplicate macro definition warnings. This change suppresses emulation of clz using _BitScanReverse if the __clang__ macro is defined.

Add fprintf overload that writes to a std::ostream

For completeness, add an overload for printf that takes a std::ostream.

Reduce compile time of variadic functions (2)

for consistency with used coding conventions.

Declarations for shared library in Windows.

Added __declspec(dllimport) and __declspec(dllexport) declarations
when compiled in Windows.