Add BasicStringHelper module
Add Fake.EnvironmentHelper.encoding

Current version FAKE always uses Encoding.Default for IO access.
Encoding.Default returns Shift_JIS using Japanese Windows.
Shift_JIS is incompatible to Unicode.
But we usually use utf-8 for source code.
So I add "encoding" build parameter.
Merge pull request #164 from hafuu/default-encoding

add "encoding" build parameter