Add support for Symbol()

[wesgarland]

Describe your feature request here.

We need to be able to send Symbols from JS to Pythonmonkey. Key characteristics:

• unique Symbols in JS make unique objects in Python
• same Symbols in JS make same objects in Python
• no requirement to create Symbols from Python although it would be cool
  • could just use python.eval('Symbol') under the hood?
• some kind of introspection would be nice, eg a working toString() in Python

It's possible the that pypi SymbolType package could be of use here.

Code example