Fixed displaying of SX, DMatrix, #982
Introducing a new CasADi interface to Snopt

-- with much love from Greg
Added SXElement::sym (replaces SXElement(string)) #983
Removed current SXElement(string) usage, #983
Updated GenericMatrix::sym overloads #983
Replaced ssym with SX::sym in the symbolic core, #983
Replaced msym with MX::sym in the symbolic core, #983
Re-enable NLP tests for other solver
Unittests for Snopt correction
Adding Snopt find_package to build system
Adding Snopt makefile to build system
Adding Snopt find_package to build system (2)
Removed current ssym, msym, MX(string) usage, #983