-Support for building with multiple toolchains.
-Fixed a bug that made --<toolchain>-architecture switches mutually exclusive when they should not be
-Changed alias for --all-targets from -a to --at
-Aliased --architecture as -a
-Aliased --all-architectures as --aa
-Added --all-toolchains, aliased to --ao
-Added csbuild.SupportedToolchains() to specify which toolchains will be used to build a project with --all-toolchains specified and disable building a project if it does not support a given toolchain
-Added method to compiler objects to retrieve their default architecture. Base class implementation returns the architecture of the system currently running the build. Android implementation defaults to arm.
-Modified the android unit test to be buildable on both android and msvc