WIP: Improving Software Carpentry Google Map

Greg raises several good points in the previous PR: https://github.com/swcarpentry/site/pull/166.
I thought I'd address them with a new PR.

**Currently Unresolved**
(i) How do I change the marker symbols on the bottom of the map? In <div id="map_canvas"...> the img src for past markers should be "http://google-maps-utility-library-v3.googlecode.com/svn/trunk/markerclusterer/images/m1.png", for instructors "http://google-maps-utility-library-v3.googlecode.com/svn/trunk/markerclusterer/images/people35.png"
In the screenshots below this is changed in _site/bootcamp/index.html only.
*The symbols on the bottom of the map are different in the screenshots* (what they are supposed to be, but I need help on how to change them.)

**Resolved by this PR**
(ii) Individual instructor pins are all the "person" symbol now, even if there is only one.

**Resolved by this PR**
(iii) To do the great spread of instructors justice, I decreased the parameter gridSize to 15. This makes it also more impressive, yet organized.
![instructors_only](https://f.cloud.github.com/assets/2601674/1335610/98eb1bbc-35bb-11e3-8c5e-84e5d157922f.png)

**Resolved by this PR**
(iv) The default is that larger clusters get a different color, hence the orange circles. That was confusing, it has nothing to do with restricted boot camps. Past bootcamp markers are now always the blue circle.
![past_only](https://f.cloud.github.com/assets/2601674/1335637/e0abb02e-35bb-11e3-9bea-f7c82024058d.png)

**Resolved by this PR**
(v) To remain the clickability of past boot camps the clusters stop at maxZoom: 6. At this zoom the numbers disappear, but the symbol remains the blue circle.
![withclick](https://f.cloud.github.com/assets/2601674/1335644/0eb6d192-35bc-11e3-8151-a90714c37d1d.png)


If you're happy with (ii)-(v) and can help me with (i), I will update the PR. I suggest to not merge this before (i) was solved.