move version string inside dtcwt package

Move back to having the version in one place. This has the dual
advantage of making the version number query-able by programs making
used of the dtcwt library.
add numpy and six to setup_requires

Since setup.py (and the docs) need to be able to import the dtcwt
package in order to read the __version__ attribute, minimal requirements
to *import* dtcwt should be added to setup_requires. This list should be
kept small.