[api] login option for application/x-www-form-urlencoded

[doublerebel]

Hello,

Cloudant requires form encoding for logins, but accepts JSON otherwise. This patch enables the form encoding and is backwards-compatible.

I had some trouble getting the tests to run locally, but I don't see this causing any regressions. Since Travis is setup for this repo I can make sure this PR passes all tests before merge.

Thanks!

[doublerebel]

As we can see in the build history something about Node 0.8 causes the tests to fail when manipulating the password. It seems the tests were never fully passing on Node 0.8 anyway, since they relied on crypto.pbkdf2Sync which wasn't added until 0.9.

I've fixed the Couch setup so that it detects Travis and adjusts permissions to successfully run in the Travis environment. Now all tests are passing, including the one added per my PR.

Thanks for your work, would love to get this merged!