[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 harvest() method of the ImageMosaicReader in order to support a Collection of files to upload.

An enum has been added for distinguishing the various harvesting behaviours: 
* on a File
* on a Directory
* on a List of Files.

For harvesting multiple files, a new ImageMosaicWalker implementation has been created.

A test method has also been added.