Optimizes 16MB test (generating less objects)
Updates tox (Python 3.3) and adds .env to ignore
Removes mapping tests from test_mongodict.py

Only MongoDB-related tests remain
Adds Python's mapping tests

Added mapping_tests.py from Python 2.7.5 and Python 3.3.2 source and created
tests/test_mapping_protocol.py to use it to test MongoDict.
Modifies README with information about codecs