Encoding suspects

specific character encoding support for commit diff, normal blob and blame view.

I have tested with Shift-JIS(cp932) source code.
This uses Encode::Guess to determine source code encoding.
It could be text processing slower but I didn't feel speed difference.

This add a config parameter "encoding_suspects" to set list of suspects.
I didn't tested the other encoding (e.g. latin1) but It should work in the same way.

Thanks,
-- Tetsuya