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'$\\mathrm{erg\\,s^{-1}\\,cm^{-2}}$'
```


The only way this would happen if someone wrote `git add astropy_helpers`, which they shouldn't unless they intended to.