Improve Gruntfile

Adds:
* Server task with live-reload
* Automatic task loading
* Configurable src and dist directories
Undo adding grunt-newer

grunt-newer restricts the watch for assemble to run when non-page files are edited (i.e., partials).
Merge pull request #16 from alejandrogarciaiglesias/master

Improve Gruntfile
Update devDeps and use Assemble 0.4.12