Fix issues with python generated code.

If you have a thrift service with the same name as a Python module, it's basically impossible to import it. This patch adds a new "package" option, ```--gen py:package=X```. If this option is present, all service imports will be package local, and the generated files will be inside a folder named after the package.

I.e., ```--gen py:package=crab``` would yield:
```
gen-py/
  crab/
    __init__.py
    service/
      __init__.py
      ttypes.py...
```