re-implemented theme watch tasks, add parallel tasks, added ability to c...

...reate dynamic parallel targets
Yeah, I was thinking that, like on Hughes, there might be a case where you want to combine some of the default tasks provided by GDT (like validate) with other custom tasks (watch:jshint, watch:css for livereload, etc). In order for that to work you have to set up a new parallel target and add tasks to it. I thought it be nice for future users of GDT to be able to have that ability simply by editing the config file and not having to write their own task file. 

Here's the config we have for parallel on Hughes:

https://gist.github.com/markdboyd/93bc733bc8d38bcaf9e6

However, I can totally see why that this might be something you'd rather just not include in GDT and let users handle on their own. It's not necessary in any way for the stuff I added to watch.js to work.