Update closure-util

The closure-util package works on Node 0.8, 0.10, and 0.11 (as of this writing), but it requires an updated version of npm to install on Node 0.8.  For the build to pass on Travis, we update npm before installing other dependencies.  For others who are installing the openlayers package, nothing special is required for Node 0.10 and above.  For users on Node 0.8, the openlayers package will only install with an updated version of npm.  This can be installed with npm itself.

    npm install -g npm # only needed on Node 0.8
Merge pull request #2197 from tschaub/deps

Update closure-util.
Add standard docs text to abstract base classes
Merge pull request #2199 from probins/abstract

Add standard docs text to abstract base classes
Take pixel size returned by the source into account

This change allows sources to handle tiles with a different resolution
than the one reported in #getTileImageSize(). This allows us to use
services that provide HiDPI tiles.