Change __exit__arg names to not be built ins
Add test to make sure assertion is raised with context manager
proof of failure case
reraise if exec_type is not none
return false instead of reraise
use assertRaises to avoid error w/ describe
Merge pull request #16 from tkaemming/assert_raises_exception

Prioritize the original exception in `assertChanged` instead of `AssertionError`.

Fixes #13