Update README and rxnorm examples
Improve `rxcui_for_name` a bit
Add `rxcui_for_name_approx` to utilize RxNav's "approxMatch" API
Make sure to return the RXCUI as string, not int
Add better comments to rxnorm_link.py
Fix argument and directionality bugs
Add `rxnav_approx_match` to call to RxNav's "approximateTerm" resource
Add `ndc_normalize` class method to normalize NDC codes

Normalization to an 11 digit number is done for all the double-dash containing formats and the 12-digit-leading-zero from the VA. If the NDC cannot be normalized this method returns None, with one excetion: the 6-4 format which is not covered in pseoudocode but I see a lot in RxNorm.
Add Sphinx documentation
Changes and improvements to rxnorm_link
Add separate rxnorm_link_run script to write JSON documents no db

This script can be used to store the "linked" rxnorm documents into a NoSQL database (currently MongoDB). Use `rxnorm_link_run.sh` to set your variables and run it.
Update README
NDC normalization: handle NDCs that only have one dash
bugfix
Update unit test for 6-4 NDC normalization
Use `drugClasses` as key for the linked VA drug classes
Add CSV option (and make it the default)
NDC must be exported as string of course
Allow to limit `rxcui_for_name` to given TTYs
Update README to reflect changes in commits from April 29