MISC: add pyx, .c, .h and other files to MANIFEST.
MISC: add pyx, .c, .h and other files to MANIFEST.

These were not being taken across and therefore (among other things) the
``make check-files`` utility was not working.

nipy/nipy-labs,bthirion/nipy,alexis-roche/register,bthirion/nipy,alexis-roche/niseg,alexis-roche/nipy,alexis-roche/nipy,alexis-roche/nipy,alexis-roche/niseg,arokem/nipy,bthirion/nipy,nipy/nipy-labs,alexis-roche/register,bthirion/nipy,arokem/nipy,arokem/nipy,alexis-roche/nireg,alexis-roche/nipy,nipy/nireg,alexis-roche/nireg,arokem/nipy,alexis-roche/register,nipy/nireg