Optimizations of Javascript and little bug corrections

Hi,

Fist of all thanks a lot for sharing this beautiful plugin with everyone, it's really useful.

I would like to contribute to your work by adding some enhancement to this plugin... It's a big change, but I tried to commit separately all the things I do in order you to be able simply to pick what you want if you don't want all of it.


Theses are the changes of this pull request :
* Suppress CTRL+M end lines in the CSS and tries to fix themes using box-shadow on images
  For example, with Twenty Twelve theme, all parts of the map is surrounded by box-shadows.
* Replaces header and footer scripts by HTML5 data description and jQuery selectors to rebuild the maps in a generic way
  This enhancement is offered to solve these issues :
 * It separates the JavaScript from the PHP and the HTML (issue #18)
 * No further use of the ids of the divs (issue #15 - the script was creating 2 script blocks for only one div)
 * There is no inline <script> tag anymore, that permits plugins like wp-minify to minify all the script
 * The script could then be deferred to raise the performance of the page
* Async load of Google Maps API Script
* Use of Static maps API to offer the admin user to :
 * only use Static maps API to render the maps (complementary to issue #1)
 * use Static maps API at first (for performance issues), and load Google Maps API on click or mouse over (issue #7)
 * keep the current behaviour (no use of Static maps API)
* Restore the overview option on GmapsV3