Fix path issues with source maps

On windows, node's path api returns backslashes for path separators.
Because the source-map library expects forward slashes in all cases,
their relative path logic (specifically their "normalize" function)
gives incorrect results when passing in backslash.  The
mozilla/source-map library won't change this, because they are actually
expecting URLs as input - see
mozilla/source-map#91 (comment).

This fix is similar to the one made for grunt-contrib-uglify:
gruntjs/grunt-contrib-uglify#175.

This should fix issue gruntjs#110 and gruntjs#95.

tasks/lib/sourcemap.js

@@ -121,6 +121,9 @@ exports.init = function(grunt) {
       // Consider the relative path from source files to new sourcemap.
       var sourcePathToSourceMapPath =
         path.relative(path.dirname(this.dest), path.dirname(sourceMapPath));
+      // sourceMap path references are URLs, so ensure forward slashes are used for paths passed to sourcemap library
+      relativeFilename = relativeFilename.replace(/\\/g, '/');
+      sourcePathToSourceMapPath = sourcePathToSourceMapPath.replace(/\\/g, '/');
       // Store the sourceMap so that it may later be consumed.
       this.maps.push([
         sourceMapConsumer, relativeFilename, sourcePathToSourceMapPath