Nicer deprecation for placeholder_tags

* Instead of breaking placeholder_tags, makes it warn.
* Documented the deprecation of placeholdr_tags
* Added a check to the `cms check` command to check for deprecations
* Added a test for the deprecation check
fixed Django 1.5 compatibility
Fixed wrong reference
fixed UnicodeDecodeError
fixed django 1.4 support
Merge pull request #2579 from ojii/nicer-deprecation-of-placeholder-tags

Nicer deprecation for placeholder_tags