Replacement for jquery.dialog to fit better with ownCloud design.

* Cleaner layout
* No resizing or moving.
* Centers in the center of container instead of browser viewport.
* Modal dialogs only disables `#content` and are dimmed instead of the ugly overlay jQuery UI makes.
* Supports a subset of jqueryui.dialog methods/options that are needed for oC.
* Option to set default button which will trigger on Enter.

To test empty browser cache and run for example:

`OC.dialogs.filepicker('test', function(val) {console.log(val)}, true, null, true);`

For a modal filepicker

or 

`OC.dialogs.filepicker('test', function(val) {console.log(val)}, true, null, false);`

for a non-modal

Also `OC.dialogs.alert('text', 'title');`, `OC.dialogs.confirm('text', 'title');` or `OC.dialogs.info('text', 'title');`

@Kondou-ger @raghunayyar @kabum @karlitschek