Make hg-fast-export (actually) work on Windows

Because on Windows sys.stdout is initially in text mode, any LF
characters written to it will be transformed to CRLF, which causes git
to blow up.  This change uses Windows platform-specific code to change
sys.stdout to binary mode.
Merge pull request #7 from living180/master

Make Windows support actually work
Rename hg-fast-export.txt to README
Update README with pointer to the github issue tracker
Fix mapping option, close #10
Merge branch 'master' of https://github.com/frej/fast-export