epub support for WP image rendering service for latex equations

Getting math equations to render in an epub output requires converting LaTeX markup to an image, and of course, including that image in the epub package. WordPress provides an image rendering service (http://s.wordpress.com/latex.php) that will convert your LaTeX equation to an image, but its URL does not conform to the pattern required in `fetchAndSaveUniqueImage()`. The lack of file extension (.png) in the url triggers an epubcheck error `remote resource reference not allowed` and appends a #fixme to the source URL. Below is an example of passing a latex equation to the WP service: 

<img class="test-image" alt="Test Image" src="http://s.wordpress.com/latex.php?latex=%5Cdisplaystyle%20P_%5Cnu%5E%7B-%5Cmu%7D%28z%29%3D%5Cfrac%7B%5Cleft%28z%5E2-1%5Cright%29%5E%7B%5Cfrac%7B%5Cmu%7D%7B2%7D%7D%7D%7B2%5E%5Cmu%20%5Csqrt%7B%5Cpi%7D%5CGamma%5Cleft%28%5Cmu%2B%5Cfrac%7B1%7D%7B2%7D%5Cright%29%7D%5Cint_%7B-1%7D%5E1%5Cfrac%7B%5Cleft%281-t%5E2%5Cright%29%5E%7B%5Cmu%20-%5Cfrac%7B1%7D%7B2%7D%7D%7D%7B%5Cleft%28z%2Bt%5Csqrt%7Bz%5E2-1%7D%5Cright%29%5E%7B%5Cmu-%5Cnu%7D%7Ddt&bg=ffffff&fg=000000&s=3">

`<img class="test-image" alt="Test Image" src="http://s.wordpress.com/latex.php?latex=%5Cdisplaystyle%20P_%5Cnu%5E%7B-%5Cmu%7D%28z%29%3D%5Cfrac%7B%5Cleft%28z%5E2-1%5Cright%29%5E%7B%5Cfrac%7B%5Cmu%7D%7B2%7D%7D%7D%7B2%5E%5Cmu%20%5Csqrt%7B%5Cpi%7D%5CGamma%5Cleft%28%5Cmu%2B%5Cfrac%7B1%7D%7B2%7D%5Cright%29%7D%5Cint_%7B-1%7D%5E1%5Cfrac%7B%5Cleft%281-t%5E2%5Cright%29%5E%7B%5Cmu%20-%5Cfrac%7B1%7D%7B2%7D%7D%7D%7B%5Cleft%28z%2Bt%5Csqrt%7Bz%5E2-1%7D%5Cright%29%5E%7B%5Cmu-%5Cnu%7D%7Ddt&bg=ffffff&fg=000000&s=3">`

This PR targets only the WP latex service as many LaTeX plugins currently use it, and for all intents and purposes could be thought of as a trustworthy domain.