Add EnvironmentConfig class

[msabramo]

Fixes: GH-16
Also takes care of the environment variable stuff I was trying to address in #11, but does it in a cleaner way.

$ SERF_RPC_ADDR=serf.company.com:6464 SERF_RPC_AUTH="secret" ipython
Python 2.7.9 (v2.7.9:648dcafa7e5f, Dec 10 2014, 10:10:46)
Type "copyright", "credits" or "license" for more information.

IPython 2.0.0 -- An enhanced Interactive Python.
?         -> Introduction and overview of IPython's features.
%quickref -> Quick reference.
help      -> Python's own help system.
object?   -> Details about 'object', use 'object??' for extra details.

In [1]: from serfclient import EnvironmentConfig

In [2]: environment = EnvironmentConfig()

In [3]: environment.host
Out[3]: 'serf.company.com'

In [4]: environment.port
Out[4]: 6464

In [5]: environment.auth_key
Out[5]: 'secret'

[KushalP]

Can you move this into a file called environment_config please?

[msabramo]

Done.