Start benchmarking browser tests.

Added benchmarking functionality:

- `log_time` decorator, which records the time a function took (applied to all subroutines).  It is tightly coupled to the test case object (couldn't find a better way) and so requires that the `Helpers` class be instantiated with a reference to the the test case.
- Writes out to a JSON file at the end of the test: if it's on the Jenkins server, to `~/.jenkins/workspace/metrics/<test battery folder>/<testname>_<timestamp>`.
- JSON object has the keys `timestamp`, `name` (test name), and `results` which is a subdict with keys as helper function names, and values as a list of the execution times of each call to it during that test.

Also extensive cleanup of `Helpers.py` (whitespace and commented-out code).