Load metadata at get_frame instead of initialize

This saves a lot of initial loading time, only loads metadata of
required frames. Also stores last frame (may be useful to arrange this
at frame.py level?)

Added omexml metadata importer, this is demanding for large files and
may require a higher java heap space.

Added direct use of the java-wrapped MetadataRetrieve routines
(mdstr/mdint/mdfloat). The latter are much faster but more information
is obtained with omexml. The java command including parameters are
passed as string. See this link for the full list of options:

http://downloads.openmicroscopy.org/bio-formats/5.0.5/api/loci/formats/meta/MetadataRetrieve.html