Better tests and fixed bug with logical operators
Use the new AST-based parser for sympy parsing. Closes #51
Make consistently use of sympy_to_str(expr) instead of a simple str(expr)
Rename parse_to_sympy to str_to_sympy (makes the name symmetric with sympy_to_str)
Add tests for sympy's syntax translation and handle some sympy oddities (equality checks, printing of and/or/not)
Merge pull request #59 from brian-team/syntax_translation

Syntax translation, closes #55
Merge remote-tracking branch 'origin/master' into language_rewrite