Added _repr_latex_() methods to Vector and Dyadic for display in IPython...

This addresses issue #2584. It adds the basic method needed to display our Vector and Dyadic objects in IPython notebooks as LaTeX renderings. But it isn't operating within SymPy's printing mechanism, `init_printing()`. This will display the LaTeX form regardless if the user calls `init_printing()`, so this may not be an appropriate solution.
![screenshot from 2013-11-17 12 06 01](https://f.cloud.github.com/assets/276007/1558777/9c1a69b0-4faa-11e3-9a79-43eaecd0b50f.png)