Version number for beta languages is updated to v3.0.0-beta-2.
Version number for alpha languages is updated to v3.0.0-alpha-5.

Protoc, C++ runtime and Java runtime are updated to v3.0.0-beta-1, other
languages are updated to v3.0.0-alpha-4.

1. make google/protobuf/stubs/pbconfig.h before making protoc, otherwise it
won't build a freshly checked-out code.
2. Document the build environments that have been tested to work.
3. Add support for MINGW64

release.

- Do not close the staging repository automatically
- Added staging.repository property
- Updated README with instructions for deployment
- Fix building 32-bit Mac artifact

Run make protoc(.exe) instead of just make, because the latter fails on Cygwin with mingw gcc; build-protoc.sh works under Cygwin; build-protoc.sh will use the --host parameter consistent with what os-maven-plugin has detected