Merge pkg_resources.Environment.__getitem__() code cleanup (pull request #47 and subsequent changes).

Correct sort order by using list.sort with support for 'key' and 'reverse'. This method is less efficient, but allows the tests to pass.

Restore install._install with a comment to capture its requirement on earlier NumPy versions. Fixes #199.

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extra : amend_source : a76d060c98e0048506e5aadddd675151db9d273c

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extra : amend_source : 4efa6b87d3acaefebdfcc953e78a452ffc1f160d

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extra : amend_source : 20ab7547c8478eb084767fe701e627bdd462ba16

Better commented the code, especially the result caching & lazy distribution
list sorting strategies implemented there. Added several TODO comments
indicating things to look into there in the future.

General code cleanup:
 - stopped reusing the project_name variable for different purposes
 - added an assertion to detect corrupt distribution list caches

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extra : source : 86ef8f59ef4e1fc253c155b9ca0856497455372d

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extra : source : 32166eed1174dc565d903c81b9d99c292357aecc

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extra : source : ab82442e2205a4ab1016711e482388590688fa15

Skip this test because it now fails under Python 3.3 and earlier apparently due to a bug in the inspect module. Since the test passes under Python 3.4 and later, that should still serve as a suitable regression test.

Extract the resolution of loader/runner classes. Allows None value to pass-through to unittest.main. Fixes #180.