Improve xkcd test coverage
Improve test coverage for calc plugin
Standardise the calc error messages somewhat
Mocked URLs aren't overwritten so easily
More test coverage for youtube plugin
Simplify youtube tests slightly
Merge pull request #82 from LordAro/remove-bitshifting

Unfortunately, bitshifting has proven far too difficult to do safely
Merge branch 'LordAro-tests'

Conflicts:
	csbot/plugins/calc.py
	csbot/test/test_plugin_calc.py
calc: make operator map more readable
Fix run_tests.py to exit with non-0 status on test failure
Disable broken youtube tests