Add line-by-line profiling option

Summary:
This ports the functionality I originally wrote for
flask_debugtoolbar_lineprofilerpanel to work with the mini profiler.

{F6708}

See the original here: https://github.com/phleet/flask_debugtoolbar_lineprofilerpanel

Coded in Oslo, Norway.

Test Plan:
With this patch applied, point your browser to your site with the mini profiler
enabled, open up one of the request profiles, then under "settings", choose
"line-by-line" as the CPU profiling option.

Reload the page. Opening up the profile details, you should be presented with
a message saying:
```
To mark functions for profiling, put this in the source of the file containing
the function you want to profile:

    from gae_mini_profiler import linebyline_profiler
    linebyline_profiler.line_profile(SomeClass.some_instance_method)
    linebyline_profiler.line_profile(some_other_function)
```

In KA's case, the profiler needs to be imported through third_party, so the
actual import becomes `from third_party.gae_mini_profiler import
linebyline_profiler`.

Choose a function that gets executed 1 or more times when loading the profiler,
then follow the above instructions to mark it for profiling. Reload your
browser.

Opening up the profile details now, you should see a table presenting the
line-by-line results.

Go back to your code and remove the profiling code. Reload the browser, and the
details should revert back to the message explaining how to mark functions.

Reviewers: ben

Reviewed By: ben

CC: alpert, chris

Differential Revision: http://phabricator.khanacademy.org/D4819