Adds default value option to get method

Gives the get function a default value argument. Which will be used if
the key does not exist.

    envvars.get('key', default='value')
setup.py pulls version from envvars.__init__
Updates readme
Updates version 0.2.0
Merge branch 'release/0.2.0'

Adds:

    envvars.get(key, default=None)