Closes #678.

Fixes #2

Previously an exception was thrown since Win32 WideCharToMultiByte API
returns error on zero-length input.

For the consumer it should not matter if fmt has been added to the
project as subdirectory or via find_package. With the alias targets
the library can be always imported via fmt::fmt.

* Gradle

Gradle support to make ease of import in Android Java projects.

* Gradle

Gradle support for Android projects.
For command line build, `gradle assembe` will trigger build phase

 - Config for Android .so file generation
 - Commented build.gradle
 - Automatic file copy after build
 - Dummy AndroidManifest file

* Build with Gradle

 - Build job for Travis CI
 - Moved gradle file to support/

* Gradle Path

 - Absolute path for Gradle binary

* File check after Gradle build

 - Additional script for build success
 - Check the Gradle's `assemble.doLast` task