Added an option to prevent certain dates using a date filter function

See issue #3
Fixed a link in the Readme
Minor readme update
Better default close, next, and previous icons

Closes issue #19
Fixed the issue with shift-tabbing out of the calendar

Closes #16
Accidentally committed unintended code

Related to issue #16
Slightly better tab behavior

Particularly when time input is disabled

See issue #16
Fixed bug with date formats in the date text input

See issue #23
References to sugar.js and date.js (commented out) in demo

Just to easily try scenarios in the demo.
Version bump
Fixes a performance issue with Firefox related to hiding the browser and click handlers

When running the date picker in Firefox, every single click in the window calls scope.$apply(), even when
the date picker isn't visible.  This causes firefox to slow WAY down.

This change makes sure that scope.$apply() only is called on click when the date picker is visible.
Works better within the context of an angular form

Fairly large refactor. Includes the following:

* Uses ngModelController to interact with ngModel
* Properly sets surrounding form as $dirty or $pristine
* Ability to add a required validation. Will set self
and surrounding form as $valid or $invalid accordingly.
* Uses standard ng-invalid css classes instead of custom ones
* Bumped version up to 1.3.0-beta
Minor cleanup and commenting
Removed the watch on ngModel

and replaced it with $viewChangeListeners and $render
Removed ng-invalid from button because it already is

being applied to the surrounding div
Fixed font family name typo

Closes issue #28
Merge pull request #27 from solarhess/fix-firefox-performance-issue

Fixes a performance issue with Firefox related to hide on click
Bumped version to 1.2.10
Fixed download link
Changed version from beta to alpha