stub for showing uploaded results finished
AbstractMetadataExtractor with list of supported extensions added
added finishing of metadata transforms
added png image test
Merge branch 'master' into stavel-master-master

Conflicts:
	differ-common/src/main/java/cz/nkp/differ/compare/metadata/external/ExternalMetadataExtractor.java
fixed new method to distinguish what extractors can be used for given file.
updated documentation.
updated documentation
added images do documentation.