At vars

[jverzani]

This PR adds a few things:

• borrows the code for @syms from SymPy to enhance @vars (allowing for arrays of variables and symbolic functions to be more easily created)
• adds a few functions that have been wrapped in SymEngine, but not brought into SymEngine.jl
• rewrites a test to avoid printing to terminal
• adds test for functions defined in mathfuns.jl
• adds tests for basic @vars usage

Sorry, I see the PR #265 is also included here. If that is not desired, I can rework.)

[jverzani]

don't know why gcd is now failing on one part of test matrix, that wasn't touched here.