Initial work on splitting bitmessagemain.py into multiple files

Most code being in the single main file (bitmessagemain.py) was raised as being a possible issue for maintainability in the future - see issues https://github.com/Bitmessage/PyBitmessage/issues/235 and https://github.com/Bitmessage/PyBitmessage/issues/200.

This pull request shows my work so far towards splitting up the main file a bit. I'm finding it a little tricky and I'd really like to hear people's opinions/feedback on this.