Type safety, regex

Features

• Made sure that evaluation fails if any parameter is a struct or complex float (neither of which can be meaningfully evaluated)
• Implemented explicit type-checking of parameters at runtime. No longer will the library panic when foo > 5 is given a string for foo, an error is returned instead. (#11)
• Implemented regex and not-regex operators (=~ and !~) which operate on strings. Implementation largely courtesy vjeantet.

Bugs

• Fixed a bug which led to a panic if Evaluate() was given a nil parameter map (#10)
• Fixed some cases where logical operators would not actually do what they said on the tin (#8)
• Fixed an embarassingly-obvious bug which prevented parsing of a close clause next to a logical operator