Allow build task to be run from anywhere and provide option for resolving relative paths.

These changes make it so the `build.js` task can be run from any directory.  It also adds support for a `cwd` build configuration option.  If the `cwd` is provided, all paths in the build config will be resolved relative to this directory.

Depends on the changes in openlayers/closure-util#27 (these need to be published first).