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.

Standardized line endings for test fixtures by enforcing LF end of files for fixtures directory
(through repository .gitattributes).  This change was needed
because the expected output of source maps are based on input files with LF, rather
than CRLF. Before this change the tests were breaking due to git autocrlf on Windows.
The difference between the source maps generated from input
files using CRLF and the source maps generated from input files using
only LF can't be normalized in the test, due to the nature of the
changes:  the offsets in the source map are changed, as well as the
embedded source.  These kinds of changes aren't as simple to normalize
as just line feeds in output files - and such normalization would be too
invasive, to the point of making the test less effective.

This should fix issue gruntjs#110 and gruntjs#95 and all unit tests should now pass.

.gitattributes

@@ -1 +1,2 @@
 * text=auto
+/test/fixtures/* text eol=lf

tasks/lib/sourcemap.js

@@ -95,7 +95,8 @@ exports.init = function(grunt) {
   // prior sourcemap and return src with sourceMappingURL removed.
   SourceMapConcatHelper.prototype.addlines = function(src, filename) {
     var relativeFilename = path.relative(path.dirname(this.dest), filename);
-
+    // sourceMap path references are URLs, so ensure forward slashes are used for paths passed to sourcemap library
+    relativeFilename = relativeFilename.replace(/\\/g, '/');
     var node;
     if (
       /\/\/[@#]\s+sourceMappingURL=(.+)/.test(src) ||
@@ -121,6 +122,8 @@ 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
+      sourcePathToSourceMapPath = sourcePathToSourceMapPath.replace(/\\/g, '/');
       // Store the sourceMap so that it may later be consumed.
       this.maps.push([
         sourceMapConsumer, relativeFilename, sourcePathToSourceMapPath
@@ -170,8 +173,11 @@ exports.init = function(grunt) {
 
   // Return a string for inline use or write the source map to disk.
   SourceMapConcatHelper.prototype._write = function() {
+    // ensure we're using forward slashes, because these are URLs
+    var file = path.relative(path.dirname(this.dest), this.files.dest);
+    file = file.replace(/\\/g, '/');
     var code_map = this.node.toStringWithSourceMap({
-      file: path.relative(path.dirname(this.dest), this.files.dest)
+      file: file
     });
     // Consume the new sourcemap.
     var generator = SourceMapGenerator.fromSourceMap(