Add initial end-to-end tests.

- Created new test runner that starts up an instance of Aspen, runs the e2e tests in the configured browsers, and tears it down.
- Created a test for the main title and strap line copy.
- Added e2e tests to the jstest make task.
Merge pull request #764 from berryp/first-e2e-test

Add initial end-to-end tests.