Changed API to account for new scope of the library

With the addition of a new reader, it was necessary to add some extra
structure to the library. The opportunity was taken to modify the API.
Removed XtoY functions from TeXMath
Updated test-texmath to reflect new interface
Updated changelog
Version bump to 0.7
Merge pull request #34 from mpickering/rearrange

API reorganisation for version 0.7