Added .pypirc authentication support for getting eggs

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extra : histedit_source : ebdc22b1f156f8af40265c5772addcfda6ae11d8
Add documentation

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extra : histedit_source : ff07b41a59ac22e1c05ce4fd42c6771378e80f13
Delint syntax

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extra : amend_source : 0f58aa917ebc71efd2904638fac3838fd0b0e4dd
Refactor for flatter code
Extract 'Credential' class for representing a username/password credential
Use 'expanduser' for better compatibilty.
Derive PyPirc from ConfigParser for more general purpose use.
Replaced __call__ with find_credential (for clarity of purpose). Removed dict_.
Get ConfigParser module from compat module for Python 3 compatibility.
Construct the 'creds' dictionary directly rather than building imperatively.
Renamed class for proper capitalization and for clarity.
Updated message when credentials used from .pypirc
Updated documentation on .pypirc support for credentials.
Merge support for loading credentials from .pypirc (based on pull request #11). Fixes #27.