Lazily load datepicker popup on open/focus

[minhtule]

This lazy loading is to reduce memory usage especially when datepicker popup is used in a ng-repeat element.

When loading 400 datepickerPopup elements (200 rows x 2 per row), it takes up 450Mb on Chrome 33 (OSX 10.9.2) and also 10+ seconds to load the page.

[bekos]

@minhtule Thx for you PR, but this will be solved when #1915 will land.

[tedeh]

This is NOT solved by #1915. Heap allocations over time in #1915:

Heap allocations over time in #2026:

This is achieved by repeatedly cycling between two different pages with two datepicker directives in them. The not-at-the-end spikes are supposed to be gray, by the way.

Any app/tab using datepicker is guaranteed to be rendered useless over time. I use up to six instances of datepicker that are rerendered from time to time and the memory usage of the browser tab shoots up to the hundreds of megabytes after a few "cycles".

Unfortunately, this pull request breaks the tests for me (even if the actual datepicker seems to work as it should which it does not do in #1915) so I'm not sure it should be merged as-is anyway.