Various changes to README.

Thought the README could use a little bit of love as well, considering it's the first thing people generally see, after all. There are a handful of changes in this PR, but there are a couple of things specifically that I'm looking for community feedback on:

### Use [Shields.io](http://shields.io) for badge images.

There's a slightly irritating problem where the default badges served by each respective service are low-resolution PNGs and don't look very nice on high-resolution screens. Using Shields solves this problem by serving SVGs; plus, they're more visually consistent.

![Badge Options](https://cloud.githubusercontent.com/assets/2434728/5335678/ca963c4a-7e63-11e4-9876-c8f16e48b157.png)

The top row is what the badges currently look like on high-resolution displays, the middle row reflects the changes made in this PR, and the bottom row is what the badges look like with the `?style=flat` suffix appended to each of the image links. Thoughts?

### Change version/downloads badges to link to [PyPI](https://pypi.python.org/pypi).

I personally think that if the information displayed in the badges both related to PyPI, it would make more sense to link to PyPI rather than a different site. That being said, if there's a reason that the badges currently point to crate.io, I can easily revert the change and rebase.