Add support for running a WebPageTest test using a script

[GoogleCodeExporter]

Today, Mobitest only supports WebPageTest tests that provide a URL to test. 
The desktop agents, however, support running a job using a WPT script as well 
(as detailed here: 
https://sites.google.com/a/webpagetest.org/docs/using-webpagetest/scripting). 

We need to add support for scripting in Mobitest as well. 

As a first step, the only script command that needs to be supported is 
"navigate", which is practically the same as getting a URL to test. However, 
the code should make it easy to add additional commands over time.

Original issue reported on code.google.com by guy...@gmail.com on 26 Feb 2013 at 3:31

[GoogleCodeExporter]

A few implementation notes:
- The script is included in the job file that the agent reads, so the first 
part would be to enhance the parser to read the extra piece of information
- You'll probably need to add some registry of script commands that will invoke 
the relevant script test function
- I suspect the most complicated part would be the async nature of invoking a 
command. When a script command completes, the code needs to know to invoke the 
next script command, instead of just uploading the results right away.
- For simplicity, it's probably ok to start with only supporting one "navigate" 
command

Original comment by guy...@gmail.com on 26 Feb 2013 at 3:35