Feature/png support

Supporting base64 encoded png output. The svgtex server now determines the output format using the URL as follows:

POST http://localhost:port/svg -> Generates SVG as it does earlier.
POST http://localhost:port/png -> Generates base64 encoded png string. 

Also Created a gem (https://github.com/quipper/capturemath) to interact with the server in a basic way.

@hakobera please review and merge with master if you are happy.