New: setting enableZoomButtons

added setting to enable zoom buttons (+/- instead of slider). Currently it is off by default.

Addresses #130
New: insert HTML for zoom buttons

mirroring zoomSlider functionality. This will insert HTML necessary for zoom buttons.

Addresses #130
New: Add HTML to label zoom buttons

adds the "Zoom Level:" label to indicate to the user the current zoom level.

Addresses #130
New: updateZoomButtons() function

added a function to update the zoom level label under zoom buttons. Mirrors existing updateZoomSlider functionality.

Addresses #130
New: Event subscription to update zoom buttons

subscribed the updating of the zoom label so that updateZoomButtons fires when the zoom level changes.

Addresses #130
New: Added zoomButtonsHTML output into toolbarHTML

createToolbar() will now add zoom buttons to toolbarHTML.

Addresses #130
New: Assets for zoom buttons

Zoom in/out icons for the new zoom buttons.

Addresses #130
New: Button mixin

class for diva buttons (including zoom in/out, grid, and link buttons), adopted from original diva-button class.

Addresses #130
New: Assets for zoom buttons

Zoom in/out icons for the new zoom buttons.
New: integrated button mixin for right toolbar css

Right toolbar icons (grid, link buttons) now inherit from the button class in mixins.less.

Addresses #130
New: zoom buttons CSS

CSS styles for the zoom buttons. The margin on the zoom in button is so that its border overlaps with the zoom out button.

Addresses #130
New: zoom buttons HTML

Creates divs for the zoom in and out buttons within the existing toolbar HTML.

Addresses #130
New: hide zoom buttons on grid view

Mirroring functionality of zoom slider hiding.

Addresses #130
Revert "New: Assets for zoom buttons"

Accidentally committed build files.

This reverts commit 439f72232b937e248ff611bd08bee1a7256ee08d.
New: demo of buttons settings

Added a demo to show usage of the new buttons settings.

Addresses #130
Fixed: controls not appearing properly

The zoom/grid buttons/slider was not appearing in the correct instances. Corrected CSS to hide all controls on page load, then we call show() in diva.js to show the appropriate controls. (Reproducing existing slider functionality.)

Addresses #130
Fixed: appropriate controls for current view were not appearing

Fixed jQuery show() calls to display the appropriate controls. (This code appears to show all controls, but only the HTML for appropriate calls has been rendered, so only those are shown.)

Addresses #130
New: enableGridControls and enableZoomControls settings

Replaced enableGridSlider and enableZoomSlider settings with enableGridControls and enableZoomControls. This is less complicated than the alternative of having 4 settings (enableZoomSlider, enableZoomButtons, enableGridSlider, enableGridButtons).
This does assume that there are no usage cases in which both buttons and slider should be displayed at the same time (cumbersome with current layout).

See discussion on #130
Doc: clarity and consistency
New: render HTML for zoom and grid buttons and button labels

Mirroring existing zoomSlider functionality. This assembles the HTML for rendering the controls in the toolbar.

Addresses #130