@named_data decorator to give tests with @data custom names

[orgadish]

#100

Changelist:

• Implemented @named_data decorator with specialty helper classes NamedDataList and NamedDataDict which get added to ddt_trivial_types.
• Implemented unittests in test_named_data.TestNamedData.

Testing:

• test_named_data.TestNamedData

[orgadish]

@wswld I tried using tox per the instructions in CONTRIBUTING.md but ran into the following error when it got to python3.5:

ERROR: InvocationError for command /Users/orgadish/PycharmProjects/ddt/.tox/py27/bin/flake8 ddt.py test (exited with code 1)
py35 create: /Users/orgadish/PycharmProjects/ddt/.tox/py35
ERROR: InterpreterNotFound: python3.5
__________________________________________________________________________________________________ summary __________________________________________________________________________________________________
ERROR:   py27: commands failed
ERROR:  py35: InterpreterNotFound: python3.5