encode name of actor to utf-8

[niyaton]

I found we cannot write log message to reflog in UTF-8 in python 2.7.5.
We can easily to reproduce this bug by executing following code to empty repository.

repo=Repo(path_to_repository) repo.create_head('test_head', logmsg='日本語')

In my environment, this code will throw following error:

UnicodeDecodeError: 'ascii' codec can't decode byte 0xc3 in position 0: ordinal not in range(128) 

I think this bug was introduced by 4a7e7a7 and 28fdf05.

I hope this PR fix this bug.

[Byron]

Thank you.
GitPython doesn't handle unicode properly, which is due to be fixed when providing support for python 3.

In the meanwhile, it shouldn't hurt to merge this PR.

[niyaton]

Thank you for quick merge!