add expressionModel from lmfit
Merge pull request #203 from licode/add_expressionModel

ENH : add expressionModel from lmfit
MNT: Made xraylib optional
MNT: Cleaned up package list in setup.py
MNT: Replaced package with the correct one
MNT: Refactor of constants.py

- Created constants sub-module
- Created api.py file in constants submodule that provides a
  clean entry point into our constants objects/functions
- Created BasicElement class that holds basic element information
- Created XrfElement class that subclasses BasicElement and adds
  X-ray fluorescence information
- Moved Xray scattering calibration stuff into xrs.py module. Will
  probably end up moving that somewhere else

MNT: Removed neutron.py from constants/
MNT: Fixed basic element creation to include more constants

MNT: Fixed link direction based on caswell comment
MNT: BasicElement now working and passing all tests
MNT: All tests passing with XRF refactor

MNT: Cleaned up imports

MNT: Fixed tests based on removal of scattering factors
ENH: Added atomic constants data
MNT: Cleaned imports and reorg calib stds
MNT: Removed constants.py. Unsure how it leaked back in
MNT: Updated based on PR comments

MNT: Fixed MRO
Merge pull request #210 from ericdill/refactor_constants

MNT : Refactor constants module to package
ENH: Implemented io api file

MNT: Added __all__ to io/api

MNT: Fixed __all__
MNT: Moved tests around
Merge pull request #217 from ericdill/io_api

ENH : Implemented io api file
ENH: Added diffraction api test
MNT: Cleaned up tests and added api tests
Merge pull request #220 from ericdill/test_refactor

Test refactor