Fix 2542

This PR fixes #2542 by first adding the capstyle and joinstyle properties to `matplotlib.patches.Patch` and setting the default capstyle of the axes spines to 'projecting'.

![test](https://f.cloud.github.com/assets/1819283/1399487/a278238e-3cb7-11e3-9ecc-6db991aa8da0.png)