TeXMath Writer: Now uses Reader and Writer monads

This change enables the threading of an environement which indicates
what LaTeX packages are in scope for the conversion. The Writer replaces
the string concatenation in order to make it easier in future to change
the underlying representation and produce more readable code.
Better escaping of converted ASCII strings
Improved single styled character handling

There are two ways to represent styled characters. One is to attach a
math variant to the ASCII character, the second is to use the unicode
character directly. This commit aligns the handling of both these cases.
Added 8519 and 8520 to the toASCII mapping
TeXMath: Blank space at end of line removed
Updated executable and test script for new interface
Updated changelog to reflect new interface
Merge pull request #37 from mpickering/master

More changes towards the release