Use swig %pythonbegin instead of PREPEND_BOILERPLATE_TO_PYTHON_PROXY_MODULE

[vitaly-krugl]

PR #1030 added PREPEND_BOILERPLATE_TO_PYTHON_PROXY_MODULE to prepend the necessary boilerplate python code and file header from src/nupic/bindings/swig_proxy_preamble.py to the swig-generated python proxy modules.

I later learned that this can be accomplished via the Swig element %pythonbegin in the corresponding .i files instead of creating custom commands during the build.

The idea is that the solution should include the shared src/nupic/bindings/swig_proxy_preamble.py contents, but not copy-paste them into the .i files. Copying manually to various .i files is bad for maintainability, so don't do it.