Substantial refactoring

This refactoring includes some PEP-8 compliance changes, as well as some more
complete testing to ensure that we're in fact serving everything out of
cassettes when we thing we are.

Incidentally, it also includes fixes for #3 and #4
add unit tests
add unit tests for Cassette implementation
fix typo
remove typo
remove fixtures, remove some dead httpbin assert code
fix typo
oops, shouldnt have removed that file
fix travis
bring back the files module, but do serialization in cassette
dont append to cassette file
replace cached() with mark_played / play_count
remove some TODOs
check that cassette contains things before returning
add back the secure write
add pytestdebug to .gitignore
add requests class to sow the seeds of a custom matcher
hold counts in a Counter
update tox to reflect new text directory structure
add backported counter module