Issue title: Add setup.py classifiers
 Issue body: On the [main Werkzeug page](http://werkzeug.pocoo.org/) it says:

> Supports Python from 2.4 up to 2.7.

However, in the 0.9 releases, there are several uses of class decorators, which are [only available in Python 2.6 and later](http://python3porting.com/improving.html#class-decorators) (decorators for functions are available in 2.4, but `@implements_…` and `@native_…` used in Werkzeug are class decorators).

I don't expect you to roll back your use of class decorators - they are nice, you should use them, and when Travis CI drops support for Python 2.5 I won't care anymore whether you support it in what version, but you might want to update the text in the main page to reflect this change in version support.  Seems like every couple of weeks another package we use is dropping support for 2.5, so you're definitely with the trend.

 Comments: 
 Comment 0: This is fixed in the git repository.

 Comment 1: Some pypi metadata about python (minor) versions would be nice, too.

 Comment 2: AFAICT there's no way to denote support for specific versions of Python 2 or 3, adding all classifiers feels ugly and I am not sure what advantages that brings. The main site is fixed, and Armin wants to keep the README small.

 Comment 3: The classifiers ARE the way to denote support for specific versions of Python 2 or 3.
So what's the problem with them?

 Comment 4: I just don't see which problem they're supposed to solve. I suppose it's just me.

 Comment 5: The classifier system is weird and I don't think it's a good idea overall nevertheless I agree with @ThomasWaldmann here, it does work very well for defining which Python versions, implementations etc. are supported or at least it's the most reasonable way to do that at the moment.

Furthermore it might be possible to generate them based on the tox configuration, so that they are always up-to-date. I'm not sure how easy that would be but it would be easier than generating documentation we clearly don't manage to keep up-to-date very well.

 Comment 6: I think we can manage to keep them up-to-date manually. The problem with the docs is that they weren't automatically updated from the repo.

 Comment 7: https://github.com/mitsuhiko/werkzeug/pull/719
 Comment 8: Fixed by @vatsaia's PR
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