[data] Add unicode.xml file

From:
  http://www.w3.org/2003/entities/2007doc/#source

Not used yet.
[data] Read entities from unicode.xml
[data] Remove now unused htmlmathml.json
[engine] Use bool here to simplify things
[engine] infiniteMaxSize is always false

Not used anywhere anyway.
[data] Auo-build the operator dictionary

Now it is built from from unicode.xml which should ensure it is kept
up to date.

Fixes #15.
[data] Remove no longer used files