[Proof of Concept] Simplfy the rules for > and <

Instead of having these operators imply a ``!=.*`` this will instead have them more-or-less simply check if the prospective version is greater than or less than the specifier version. They will not however match a post or pre-release of the version mentioned in the specifier unless the specifier itself contains a post or a pre-release.

In particular that means that something like ``>1.7`` will match ``1.7.1`` but will not match ``1.7.post1`` while ``>1.7.post0`` would. Likewise ``<3.0`` would not match ``3.0.dev0`` while ``<3.0rc1`` would.

This PR also includes two new attributes on ``Version`` instances: ``is_postrelease`` to determine is a particular release is a post release and ``release`` (Is there a better name for this?) which will return just the part of the version which shows the "release" version (e.g. ``1.0`` is the release version of ``1.0.dev0`` and ``1.0.post0``).

This requires an update to PEP 440 as well as documentation and tests before it can merge.