Create filesystem crawler

[ScottWales]

Create a crawler that will recursively scan a directory, loading the paths it finds.

If run a second time on a directory it should add only newly created files

[ScottWales]

metadb collection create --collection foo /root/path /another/path
metadb collection crawl --collection foo

should crawl all files under /root/path and /another/path, adding them to collection foo.

metadb collection crawl

should crawl all collections