Fix encoding problems in form fill and POST requests.

This PR fixes encoding problems:
- Fix submitting forms filled with unicode strings containing umlauts (mechanize problem)
- Fix making requests with unicode payload containing umlauts (urllib2 problem)
- Support filling forms with utf8 bytestrings instead of unicodes (automatic conversion)

Closes #5 

@Tschanzt I'm not sure whether this fixes your issues because I did not have the same traceback in tests.
Could you checkout my branch and run your tests on it?