Merge branch 'pu/parse'

Fetching Root from pu/parse
Merge branch 'pu/parse'
Eval clause for roots
UPN tests added
Now using native Show instance for terms.

Traditional show instance renamed as `showTerm :: SymTerm -> String`
Fallback clause in showTerm
Travis CI configuration added
Started implementation of TermToTex.

This new module is able to convert SymTerms to LaTeX math code.
Fixed cabal build
Completed LaTeX output module
Improved version specification in cabal file
Replaced old UPN parser by a new Parsec-based one
Adapted Travis build icon
upnToTerm modified to return (Either String SymTerm).

This is necessary for the parser unit tests to be executed.
Power operator had been missing.
Linked Travis page
Better error handling using fail

`unexpected` might not be the best for this case :/
Bug fix: Wrong simplification rule for Roots
Module rename: UPNParse -> RPNParse
RPNParse documentation added.