Add "latex_inline" unit format representation with negative powers instead of fractions

This PR adds a new unit format representation called ``latex_inline`` which uses negative powers instead of fractions for expressions of rate. It is otherwise equivalent to the ``latex`` format. This format is the one used  in the [IAU Style Manual](http://www.iau.org/static/publications/stylemanual1989.pdf) and required by some publications (e.g., [ApJ and AJ](http://aas.org/authors/manuscript-preparation-aj-apj-author-instructions#_Toc2.2)).

```python
>>> fluxunit = u.erg / (u.cm ** 2 * u.s)
>>> fluxunit.to_string('latex')
u'$\\mathrm{\\frac{erg}{s\\,cm^{2}}}$'
>>> fluxunit.to_string('latex_inline')
u'$\\mathrm{erg\\,s^{-1}\\,cm^{-2}}$'
```


Unlike the latex example above, this won't actually be the default representation in the IPython notebook (we could change that in the future based on discussion, but at present that's not the case).  So this should read:

```
>>> fluxunit.to_string('latex_inline')
```