Fix indentation

This makes it clearer where the if statement begins
Move print_buffer functionality to global scope
Print "/otr log" error messages to correct buffer
Use user-configurable prefix for output

Fixes #51
Colorize messages according to type
Merge branch 'colorize-output' of https://github.com/tribut/weechat-otr into tribut-colorize-output

Conflicts:
	weechat_otr.py
Merge branch 'tribut-colorize-output'
Make sure string_eval_expression is called in tests.
Add colors to mock weechat.
Use () instead of + for multiline strings in tests.
pylint style recommendations.
Change prefix to :! otr !:.