Fix jsonp content-type response header

Incorrectly sends application/json rather than application/javascript on a jsonp request which breaks strict mime handling. In response to error: Refused to execute script from '...url...' because its MIME type ('application/json') is not executable, and strict MIME type checking is enabled.
Offload WordPress installation to bash script so it can be reused
Run tests against single site and multisite
Support for running tests in a few ways
Change our plugin activation test
Improve tests for multisite
Check that the JSONP callback is a string first

If you pass in an array, this would cause a warning.

Props @ircrash
Merge pull request #405 from WP-API/double-check-jsonp-format

Check that the JSONP callback is a string first
Add newline
Different type, per Ryan's request
Add steps to readme for executing tests with vagrant ssh -c
Merge pull request #380 from simonlampen/master

Fix jsonp content-type response header
Merge pull request #410 from EdHurtig/master

Match dashes in addition to alphanumeric and underscore
Merge pull request #416 from kadamwhite/testing-from-outside-vagrant

Add steps to readme for executing tests with vagrant ssh -c
Merge pull request #397 from danielbachhuber/flexible-tests

Flexibly run tests