Halibot should respect module/agent requirements.txt

[richteer]

Halibot core has few to no python module dependencies, but modules may require extra libraries. We should allow some form of signalling when a halibot-module requires a python-module. For example, if the user tries to install the halibot-irc agent, and do not have pydle installed, we should at the minimum throw a warning. Bonus points to also allow fetching of the requisite python module via pip. Fetching dependencies should not be a default action, however.

Breakdown:

1. Warn the user if a halibot-module requires an unresolved external dependency
2. Support version checking and version enforcement for external dependencies
3. (extra) Support optionally fetching dependencies