Add ext and ext_clean rules to Makefile.

The default is to build the Cython extension. The Cython extension
is always deleted and re-built since Cython isn't too good at
detecting when it needs to be re-built.
Merge pull request #145 from OP2/makefile-build

Add ext and ext_clean rules to Makefile.

The default make target builds the Cython extension. The Cython extension
is always deleted and re-built since Cython isn't too good at detecting
when it needs to be re-built.