fix(gridster): remove_widget Returns the instance of the Gridster Class

Following same pattern as remove_all_widgets --> remove_widget also
return *this* (as per inlined comment documentation)
fix(gridster): Orphan preview holder when dragging is interrupted

After beginning a drag, if the mouse button is released outside of the
screen, and if a subsequent drag operation is begun on another widget
(while the previous widget still follows the mouse around), Draggable
will trigger a new drag start event, causing Gridster to leave the
current widget + its preview holder aside and start dragging the new
widget. This will lead to an orphan preview holder to be left on the
screen.

A quick solution is to ignore drag start without receiving a drag stop.
fix(gridster): overlapping widget problem
fix(gridster): the preview holder should not always use `li`

Use the same element as the original widget for the preview holder.
Using a <li> can cause a list item bullet to be displayed when widgets
are not necessarily <li> elements.
fix: fixes and improvements in widget-resizing. Closes #32
style(generated-css): add line breaks in css declarations
chore(grunt): add grunt-bump to manage releases
chore(grunt): add grunt-conventinal-changelog
feat(utils): add delay helper to utils
feat(draggable): pass useful data to all drag callbacks

start/stop/drag callbacks receive: element offset, cursor position,
and references to $player and $helper elements.
chore(css naming): `gs_w` to `gs-w` following CSS naming conventions

BREAKING CHANGE: If you are using `gs_w` classes somewhere, replace
them with `gs-w`
chore(draggable): add default `helper` config option for clarify
feat(draggable): Don't start new drag if previous one hasn't stopped

Moving 4e6159fb by @atesgoral into draggable.js
feat(draggable): pass previous position to the drag callback
feat(draggable): CSS selectors support in `ignore_dragging` config opt
feat(draggable): new config option to move or not the dragged element
chore(draggable): jQuery adapter returns Draggable instance

Draggable instance instead of a jQuery collection.
Why? Becomes a mess if Draggable is instantiated multiple times from
the same HTML element.

BREAKING CHANGES: If you was accessing to Draggable instances through
.data('drag') you should remove the `data` getter. Since now the instance
is returned directly.
style: some whitespaces, line-breaks...
chore(grunt): add css files to grunt watch command
feat(gridster): drag-and-drop widget resizing