Bundle Python dependencies

The preferred way of handling Python dependencies in Homebrew is to
bundle them in the package's libexec directory.

This change does not include matplotlib due to a build issue which is
still being investigated.
Style fixes

Homebrew upstream have become quite strict regarding the use of
apostrophes over quotation marks.  This change cleans up the GNU Radio
formula a bit to follow this style.
Fix GNU Radio Companion blocks path

Unless specified, GNU Radio Companion will look for blocks in its
install prefix preventing it from picking up third party blocks
installed on the system.