Replace parser with MathTeX.NET one

A better parser infrastructure for TeX syntax already exists in the [MathTeX.NET project](https://launchpad.net/mathtexdotnet), which I worked on around the same time as the original version of WPF-Math. I don't mind using an alternative, but I do think we should refactor the parsing code into a proper, extensible project in another project, and have the core layout/rendering library only take an AST (abstract syntax tree) as input. That way we can then add other input syntaxes in a nice way (e.g. MathML). Of course, the rendering will later be split off from the core library too, but that can come later.