Preserve backslashes in _.template()

For example, templates containing things like the literal string
`\ribbit` weren't being escaped properly when being compiled. The `\r`
was being interpreted as a carriage return character.

I've added a test for this too, and all the existing tests pass.

underscore.js

@@ -646,7 +646,8 @@
     var c  = _.templateSettings;
     var tmpl = 'var __p=[],print=function(){__p.push.apply(__p,arguments);};' +
       'with(obj||{}){__p.push(\'' +
-      str.replace(/'/g, "\\'")
+      str.replace(/\\/g, '\\\\')
+         .replace(/'/g, "\\'")
          .replace(c.interpolate, function(match, code) {
            return "'," + code.replace(/\\'/g, "'") + ",'";
          })