Add split_string_after to errbot.utils
Split test_execute_and_send test into multiple seperate tests

This makes debugging a little easier if a part were to fail
Move test_execute_and_send tests into a seperate test class
Split output which is too long for one message into multiple messages

This will allow plugins to return output that doesn't fit into a single message,
and leave it up to the bot framework to spread it out across multiple messages
instead.

Implements and closes #115
Allow configuration of MESSAGE_SIZE_LIMIT
Merge pull request #156 from zoni/issue/115

Implement feature request from issue #115 (Split messages that are too long)