Initial Android support

[enfipy]

Closes #87

I was able to run shaderc-rs on the Android app with dynamically linked shaderc that was built with the ndk-build tool inside build/shaderc/android_test folder with the following command:

ndk-build APP_BUILD_SCRIPT=Android.mk SPVTOOLS_LOCAL_PATH=../third_party/spirv-tools SPVHEADERS_LOCAL_PATH=../third_party/spirv-headers APP_STL:=c++_shared APP_ABI=all -j

Then I provide the path to the libs folder as SHADERC_LIB_DIR. I'm sure that this is not a good way to make it work but still - it works and I want to hear any feedback on how to make it right.

Also, there's still not possible to build it with the CARGO_FEATURE_BUILD_FROM_SOURCE and without PITA while building it with ndk-build by yourself. Should I add ndk-build to build.rs or there some way to do it with ninja?

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[antiagainst]

Thanks for adding this!! Could you rebase first? The buildbots failure is fixed at top of the tree. I'll take a look later. :)