improve grunt watch task

[leobalter]

In a private chat I saw @jzaefferer's will to improve speed on our grunt watch task.

Current options to try (not exclusive):

• grunt-contrib-watch spawn and interrupt options.
• grunt-concurrent
• grunt-newer

Some goals:

• grunt watch needs to be faster
• don't rebuild src files when only test files are modified
• can we use those tools to run jshint/jscs only at the modified files (src or each test files)
• run tests in parallel (if this goes faster)

[rdugue]

I made some changes on a branch to run jshint/jscs only modified files but it is actually making grunt watch run ~2 seconds slower for me. You can see the change I made here. I'm going to test some more.

--edit: grunt watch seems to run significantly faster with spawn set false and interrupt set true.
--edit 2: Running tasks with grunt-concurrent seems to also improve speed.

[leobalter]

Closed by #886