ENH: Make setup polite when checking NumPy, SciPy and Pandas

Modification to setup so that it will refuse to upgrade old versions
of NumPy, SciPy and Pandas if encountered
Merge pull request #7 from bashtage/polite-setup

ENH: Make setup polite when checking NumPy, SciPy and Pandas
ENH: Improve str and repr for all models

Models should have meaningful str and repr
Merge pull request #8 from bashtage/improve-repr

ENH: Improve str and repr for all models