Add curation capabilities to the database

[pagreene]

The PR sets up the database, client, and rest api to handle posting and (in the case of the client) getting curations. Specifically, the PR:

• Adds a curation table for both PA and Raw Statements.
• Adds a function to submit curations to and get curations from the database
• Adds an endpoint of the rest api to submit curations

In addition, the PR:

• Refactors indra_db.client into a directory of files. This has no effect on code due to the use of the __init__.py file, however makes navigating the code much easier.
• Refactors some of the rest api endpoints to be more navigable and consistent.
• Fixes some instances of 80-character lines that should be 79 character lines.

[bgyori]

Nice!

[pagreene]

It's not clear to me why the Travis tests are getting import errors. It works locally. I'll investigate deeper tomorrow.