Distiller versioning - fix dual version strings

Two strings represented the library version: one in distiller.__init__.py
and one in setup.py.
This can lead to two difference version string values.
The fix: have distiller.__init__.py read the version string from the
package installation.
This assumes that we've installed distiller properly, but we've been
making this assumption for a long time in our code (e.g. how we do
imports of distiller from the `tests` directory).