Iterate over available conversions when a message is made.

Previously when a message is made the default conversion was used,
unless otherwise specified.  Now we will iterate (in reverse order)
over the available conversions and use the first (i.e. latest)
conversion that is capable of encoding the message.
Changes _conversions from dictionary into a list.
Asserts no longer apply.
Cleanup for get_conversion().
Documentation and assert cleanup.
Clarified docstring.
Split get_conversion(USING) into three methods.

- avoid confusion with the property USING
- use similar schema as get_member, get_member_from_id
Import not needed outside __debug__.
Fixes typo in comment.
Merge pull request #98 from boudewijn-tribler/iter-conversion-on-message-creation

Improvements to conversion selection.