Adds first pieces of MascSlab and one MASC document to test it with.
Adds an annotation for the MASC source URL
Adds support for reading sentences from MASC -s.xml files
Adds support for reading segments from MASC -seg.xml files
Adds support for reading part-of-speech tags from MASC -penn.xml files
Adds support for reading named entity mentions from MASC -ne.xml files
Merge pull request #22 from bethard/masc-slab

Adds support for loading MASC annotations into a Slab (issue #19)