[MJ-775] Partner library Backend

Introduces the PartnerLibrary model as well as tools to manage it:

* `python manage.py update_partner_library` - parses HTML returned from the OFCCP directory after some trickery (thanks @almathew) and converts the result into PartnerLibrary records, discarding duplicates (as determined by email, city, and state).
* get_library_as_html - returns a string representing the html returned by the fore-mentioned trickery
* mypartners.helpers.get_library_partners - generator that returns a lightweight object representing a partner retrieved from the OFCCP souce mentioned above. This function drives the `manage.py` command. This can either parse an HTML text file, or retrieve HTML if you have a working connection.

The PartnerLibrary model omits the job category fields since it was determined that they weren't necessary for my use case. However, the helper functions retrieve that data, so should we find a use for them (such as tags), it would be a simple matter of adding a migration along with `BooleanField`s to represent them.

@galitskyd's work is starting to shape up, so I thought I'd push this to make it easier to collaborate.