Handle exit messages from tests in ex_unit

Fixes #982
Implement Keyword.delete_first, closes #984
Merge pull request #985 from ericmj/exunit_exit

Handle exit messages from tests in ex_unit
Merge pull request #986 from meh/keyword-delete_first

Implement Keyword.delete_first
Merge pull request #980 from meh/dict-equal

Implement Dict.equal?
Return early in Dict.equal? if the dicts have different size