Grunt: generate source maps when compressing
Strip build prefix from sourceMappingURL (see #51)

Previously the sourceMappingURL would be prefixed with
"build/openseadragon/"
Grunt: normalize build paths

The sourcemaps included build/openseadragon/ in the paths. Since grunt
lacks a standard way to replace text in files, I semi-randomly picked
grunt-text-replace.
Update to grunt-contrib-uglify 0.2.2
Merge pull request #51 from acdha/generate-source-maps

Generate source maps