Add installation documentation for module underscore

[baaskoen]

If I'm following the installation documentation and try to configure Node Query Builder, I'm getting the following error:

Error: Couldn't load the Connection library for mysql({"user":"root","host":"localhost","database":"myDatabase","password":"myPassword","pool_size":50}): Error: Cannot find module 'underscore'

Perhaps it is a good idea to add npm install underscore to the documentation?

Great work though 👍

[malles]

IT should be added to the dependancies.
The tests on travis also fail because of missing underscore

[kylefarris]

I was only using underscore for its extend method. Node has had the native es2015 Object.assign method for some time now so I've removed the underscore dependency and changed all instances of _.extend to Object.assign. That does mean, however, that Node version >= 4.0 is now required (was 0.10). I don't think this is too much of a stretch, though, seeing as version 7+ is out now.

Travis builds are now passing.