Added ability to force reveal side in xblock markup

This patch allows to explicitly set on which side hotspot popup must be shown.

Affected components:  xblock-image-explorer
Affected users: backward compatible with existing course setups. Opt-in to override behavior.

Test instructions (`...` means leave other attributes as is):
1. Import course tarball from https://gist.github.com/e-kolpakov/6a60cd413fe5143dcba7#file-imageexplorerpopupoverride-tar-gz
2. Log in to studio using staff@example.com / edx
3. Navigate to only unit in the course
4. There should be two image-explorer Xblocks: Full auto and Overrides.
5. Full auto should show both popups to the right of the hotspot
6. Overrides should show top hotspot's popup to the right, bottom hotspot's popup to the left.
7. Edit Overrides Xblock so that `<hotspot>` elements have their `side` attributes  changed:
7.1. Set first (hotspotA) `<hotspot ... side='right'>` - should display first hotspot's popup to the right
7.2. Set second (hotspotB) `<hotspot ... side='left'>` - should display second hotspot's popup to the left
7.3. Set first (hotspotA) `<hotspot>` side to anything except `left` or `right` - should display first hotspot's popup in automatic mode (if x and y haven't changed - to the right)
7.4. Set first (hotspotA) `<hotspot x='600' y='20' ... side='right'>` - should display popup to the right, despite crossing image border
7.5. Set first (hotspotA) `<hotspot x='100' y='20' ... side='left'>` - should display popup to the left, despite crossing image border

![xblcok-image-explorer-popup-side-override](https://cloud.githubusercontent.com/assets/3330048/4197676/ab291cf4-37ed-11e4-84a7-36636fc56393.png)