Run each test in a new bot thread

This provides more isolation and makes it a lot easier to debug
problems, as all captured logging pertains to one test and one test
only.

Closes #124
Split up test runs

For some inexplicable reason, the webhooks tests were consistently failing
when run as part of the whole testsuite, but passing fine when running only
tests/webhooks_tests.py on their own. Although ugly, by splitting these up
and calling each file with tests in it's own nose.run(), we can work around
this issue and have working tests again.
Do TLS certificate checking in XMPP backends
Merge pull request #181 from zoni/isolated_tests

Run each test in a new bot thread
Merge pull request #180 from zoni/issue/175

Split up test runs
Merge pull request #182 from zoni/verify_tls_certs

Do TLS certificate checking in XMPP backends