Fix HTTP request parsing mechanism

[asmecher]

Currently TextureHandler contains code to manually parse an HTTP multipart request. However, using file_get_contents('php://input') with multipart/form-data is out of spec. From http://php.net/manual/en/wrappers.php.php:

php://input is not available with enctype="multipart/form-data".

While it does appear to work on some systems, it's been reported not to work on all. Another mechanism needs to be used to communicate with the Texture editor. (Ideally, we should not be manually parsing HTTP requests at all!)

(Note that #6 may require a rewrite of the storage interface anyway -- this should be considered in conjunction with that issue.)