call done() exactly once

fix for grunt.async().done() being called multiple times
(for each source file), which broke folloing grunt async tasks.
bumping verion number
added preEncodeCallback - allow client to tune image embedding

callback can return
	- 'true' to continue with default embedding,
	- 'false' to abort grunt task
	- String to replace the image with
Merge pull request #31 from gariev/master

call done() exactly once