Fix __version__  

Cleaning up the ` __version__` to be the right version in `__init__.py` uncovered a few places where the version was being used, specifically in `warn_deprecation`.

I am using `pkg_resources` to grab the version of the installed package rather than requiring manual intervention to keep the two in sync. The only place that I think it is being used though is in `warn_deprecation` so it might be possible to remove that logic into that location and remove the `__version__` entirely from `__init__.py`. 

I modified `warn_deprecation` so that on version 1.3, it now raises `DeprecationWarning`, which caused a few tests to fail (as now a DeprecationWarning was being raised) I've since then modified the tests.

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The only commit that I am not sure of is: https://github.com/bertjwregeer/webob/commit/546e82e59787c66a054189de9886eea66e18cf59 and I may revert it and simply bump the TODO to version 1.4 to fall in-line with all the other `warn_deprecation` that are tagged version 1.3