Adds code to deal with exception rules. Fixes #6

Exception rules were being ignored.  Added test cases from Mozilla (provided by
Rob Stradling of Comodo), found here:
nmxr.mozilla.org/mozilla-central/source/netwerk/test/unit/data/test_psl.txt?raw=1

Updates DomainParserTest to correct improper assumptions as a result of ignoring
exception rules.