Don't import from astropy before it's installed
Don't try to import poppy before it's installed
Declare dependencies for WebbPSF in setup.py
Warn of weird NumPy setuptools interactions in setup
Make explicit the dependency on scipy and matplotlib
Update astropy_helpers to v0.4.2
Updated installation docs with an eye to outside-STScI users

  - Fixed broken links in Pysynphot installation instructions
  - Formatted the list of dependencies nicely
  - included a tl;dr quickstart at the top
Little more rearranging of installation.rst
Added conda install instructions
Merge pull request #19 from josePhoenix/master

Improve robustness of setup.py in the absence of certain dependencies