[GEOS-6477] REST Upload via POST on a Mosaic could be more performant

Please look at the following JIRA: http://jira.codehaus.org/browse/GEOS-6477.

This pull request modifies the StoreFileUpload.doFileUpload() method in order to return a List of files.

When the request is a PUT, then the List contains only a single file to use for creating the store. When the request is a POST then the LIst contains only the new files to add to the Store.

Also a test has been added.

Note that this pull request requires to merge the https://github.com/geotools/geotools/pull/446 in GeoTools.