Cache .so files containing debug symbols.

Summary:
The `extract_symbols` step on buildbot has been broken for a long time because
whenever we hit a cache hit for ndk libraries, the script cannot find these
files with symbols.

Test Plan:
buck build instagram
buck clean
buck build instagram

`find buck-out -type f -name '*.so' | grep local` is not empty.
Add documentation for python_library and python_binary rules

Summary: As per title.

Test Plan: ./docs/soyweb-local.sh
Fix `command -v` calls in buck_common.

Summary: The existing watchman test always returned true for me on Ubuntu 13.10.

Test Plan: Worked on Ubuntu 13.10.