Fix google map when rendered hidden.

I have applied this fix to the google maps block:
http://www.concrete5.org/documentation/how-tos/developers/improve-rendering-of-jquery-plugins/

Most users see no difference because the map block is visible and therefore starts straight away.
If the map block is inside a hidden element, then the map will be started only once the element becomes visible.

The consequence is that the map block now has a visible container of known size and the map will always 
render cleanly.
Merge pull request #775 from JohntheFish/patch-6

Fix google map when rendered hidden.