Fix -Wsign-compare warnings in int32_size()

From https://developers.google.com/protocol-buffers/docs/proto#optional:
If the default value is not specified for an optional element, a
type-specific default value is used instead: for strings, the default
value is the empty string. For bools, the default value is false. For
numeric types, the default value is zero. For enums, the default value
is the first value listed in the enum's type definition. This means care
must be taken when adding a value to the beginning of an enum value
list.

Prior to this change, protoc-c set the default enum value to 0, whether
or not 0 was the first value listed in the enum's type definition (or if
it even was listed at all).

Using the old "protoc-c" command directly is still supported
through a symlink.

Using {} as an initializer fails on MSVC
Signed-off-by: Paolo Borelli <pborelli@gnome.org>

I've moved the guide to its own domain some time ago, fix the reference to avoid the redirects.

Remove a redundant test and add two large number test cases.

This change helps keep protobuf-c compatible with systems where int is
less than 4 bytes long. This change also makes left shifts more
consistent.

Fix a few casts where ints were cast to uints unnecessarily

Fixes #199. Previously, enums were treated as uint32's, but they need to
be treated as int32's instead.

t: Add a few test cases with negative enum values

Add support for multiple versions of GCC and clang by using a build
matrix. Switch Travis VM to be based on Ubuntu 14.04

Adds support for the LITE_RUNTIME optimization option to the protobuf-c
compiler. Enabling this option would generate lighter weight message,
enum, and service descriptors that contain NO strings. As a result,
calls to lookup descriptors via the *_get_{field,value,method}_by_name
API will return NULL.

Default compiler behavior (when optimize_for is not specified or is not
set to LITE_RUNTIME) is unchanged.