Add "String to SymTerm" parser based on Parsec
Implement derivation
Add parser for Ln
Parser: Ignore whitespace
Beta: evalTermP evaluates a term containing Variables

Using an association list
A bit of documentation and some trivial transformations
Added abs() clause
Merge branches 'pu/parse', 'pu/eval', 'pu/simplify' and 'pu/derivate' into next

Test release