Log Message:
update to 0.7.6.1
changes:
* Add QuadraticFormula, CubicFormula, PolynomialRoots, CommutationMatrix,
  ToeplitzMatrix, MakeVector, RiemannZeta, GammaFunction
* Add --exec option to execute an expression from the command line
* Add chapter to the documentation on plotting and other minor updates to
  the docs
* Fix complex number handling in number of functions, particularly in
  all functions that use gaussian elimination and related routines
* Translations updated a teeny bit
* Fix rational powers