Another test case written, another bug squashed

Turns out zKillboard changed their URL format.
Add test for zKB characters not in an alliance
IDs should be ints, even if zKB gives us strings
Add positive CRESTMail test
Use more specific assertions

I While I don't support 2.7 at the moment, one of the goals of writing these tests is for them to be used when 2.7 support is added. I finally read the 2.7 TestCase docs, and the richer assertions were added in 2.7, not 3.1 (well, probably both at the same time).
Fix typo in invalid zkb url exception
Test zKB and CREST killmail error conditions
Transformers' default for non-given tokens should be empty strings

Found while tests were being written.
Reorder imports in test_killmail
Add transformer tests