Make cern a namespace package

If splitting the package (#30) and keeping the subpackages in the cern
namespace, we need cern to be a namespace package. Only then, multiple
independent modules can be installed into the same base package.
Move _checkCommand to cpymad.model

This function is only useful for this module. No need to keep it in a
separate location.
Remove future feature absolute_import from libmadx.pyx

This should fix the build error on the testing servers:

    cern/cpymad/libmadx.pyx:15:23: future feature absolute_import is not defined

The issue is probably caused by an old version of cython<0.18.
Use .pxd declarations in .pyxd implicitly

According to the documenation using a cimport for the declarations in
the corresponding .pxd file for a .pyx file is not necessary, see:

http://docs.cython.org/src/reference/language_basics.html#cimport

This also seems to fix the warning:

    missing cimport in module 'cern.cpymad.libmadx': src/cern/cpymad/libmadx.pyx
Fix warning: src/cern/cpymad/libmadx.pxd:64:18:

Non-trivial type declarators in shared declaration (e.g. mix of pointers
and values). Each pointer declaration should be on its own line.