Improve handling of error responses from Bing

Previously goog.asserts.assert was used to check that the response was
successful, which are stripped from compiled code.  This commit sets the
ERROR state on the source instead.
Merge pull request #1149 from twpayne/bing-maps-errors

Improve handling of error responses from Bing
Stop mspointer events

Stop mspointerup and mspointerdown events reaching the "stop event" overlay container.
Check resources length of first resource set (see #1149)
Use permanent Bing Maps API key

This key is registered for "Not-for-Profit" use by OpenLayers as a project of OSGeo - a 501(c)(4) organization.

See the complete [Terms of Use](http://www.microsoft.com/maps/product/terms.html) under the Eduction or Non-Profit Organization Use section.
Merge pull request #1150 from tschaub/bing

Use permanent Bing Maps API key.
Some additions to View2d docs
[ci skip]
Merge pull request #1151 from probins/viewdocs

Some additions to View2d docs
Export ol.extent.createEmpty function
Merge pull request #1159 from twpayne/export-create-empty

Export ol.extent.createEmpty function
Add default stability level to docs of exported things.
Fix error in map docs
[ci skip]
Merge pull request #1171 from probins/mapdocs

Fix error in map docs.
Merge pull request #1163 from pagameba/doc-stability

Add default stability level to docs of exported things.
Merge pull request #1147 from elemoine/issue-1147

IE 10 Click on popup
Small addition to view2d docs
Merge pull request #1177 from probins/viewdocs

Small addition to view2d docs.
Fix getImageElement calls in canvas and webgl renderer
Merge pull request #1184 from oterral/get_image_element

Fix getImageElement calls in canvas and webgl renderer
Add ol.coordinate.format function

Return a formatted coordinate. Example usage:
  > ol.coordinate.format([0.01 ,12.14], "{x}, {y}", 2)
  "0.01, 12.14"
  > ol.coordinate.format([0.01 ,12.14], "lat: {y} / lon: {x}", 1)
  "lat: 12.1 / lon: 0.0"