topleft/topright options, compact style, LESS

Add topleft/topright options (opens: 'topleft' or 'topright') for
positioning
- As a result, position is updated when months change
Add new 'compact' style for tighter display (option compact:true)
Add .less file to generate datepicker.css using bootstrap
variables/mixins for seamless integration
- Included mixins.less and variables.less from bootstrap 2.2.2