Fix test suite on WIN32
note: CDRIVER-126
CDRIVER-127 add env for win32
minor: fix build and tests for OSX
minor: fix warnings and compile on linux
minor: update build docs
Add simple platform test for env_posix.c
RELEASE v0.5.1
Merge pull request #38 from noa--/pullreq4

Initialize errstr in bson_empty.
minor: cleanup
CDRIVER-79 minor: fix copyright
Add env_posix test for getaddrinfo and read timeout.
minor: fix test on 1.8.4
Refactor last error storage and handling.
minor: clear warnings
CDRIVER-133 gridfile_write_buffer() fix.
CDRIVER-128 support getaddrinfo on Windows
CDRIVER-130 report WSAGetLastError()
CDRIVER-128 CDRIVER-130 tests (forgot to commit)
minor: rename errorcode to errcode for system errors.