Add support to parse and save multiple files. fixes ehynds/grunt-image-embed#19

If `dest` param is a directory — each input file will be saved into separate file in `dest` directory (with same name). Add `cwd` param to add base path for input files.
Remove files after full processing (all css files) to prevent an  error "File not found" when `deleteAfterEncoding` is `true` and one file is used in different css.