FORCE NOT NULL and FORCE NULL support

[rdmurphy]

This resolves #44 and adds support for force_not_null and force_null options to CopyMapping.

I followed the same format as the other extra_options — but these two are a little bit more complicated because they need to support the passing of lists. Let me know if you think it should be a bit more robust!

I was able to reuse one of the other test fixtures for force_null, but none of the mocks fit what force_not_null is trying to accomplish, so I created a new mock and CSV for it.

I also think I correctly updated the documentation too, but I have little experience with Sphinx so I was kind of winging it. 😬

I've been testing this against an internal app and it solved my original issue. 🎉

[rdmurphy]

I have no idea how Travis CI decides Postgres needs to be available (at least according the docs, you'd have to pass something in to services, but this project doesn't), but if I were to hazard a guess at why this failed the tests it's because it's testing against a version of Postgres that's too old to have support for FORCE NULL. That doesn't show up until Postgres 9.4. Travis CI apparently defaults to Postgres 9.1 if you don't specify.

[palewire]

I've just added the following to the Travis config file in your fork. Let's see how that does.

addons:
 postgresql: "9.4"

[coveralls]

Coverage decreased (-5.6%) to 94.423% when pulling cdd951a on rdmurphy:force-null-support into 6ab0b87 on california-civic-data-coalition:master.

[palewire]

Looks like it's passing now that I made that change. Let's merge it.