Address Greg's comments on https://github.com/swcarpentry/site/pull/166

Greg raises several good points about https://github.com/swcarpentry/site/pull/166.
I thought I'd address them with a new PR.

(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.)

(ii) Individual instructor pins are all the "person" symbol now, even if there is only one.

(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)

(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)

(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.