use versioneer for version info, remove  numpy imports from setup.py

This PR uses versioneer to set the version number.  Two versioneer-specific files are added (versioneer.py, lmfit/_version.py), but the usage within setup.py is quite straight-forward, and version numbers will be automatically generated from git tags at `python setup.py build` time.

It also removes any imports of third-party modules (numpy, scipy) that can break determining dependencies for package managers.