Add bnf loader

[kkdavis14]

not a BNF loader, actually.
new file: external documentation markdown.

edited:
base loader for initial functionality with BNF & FAST XML files. will need further testing with BNF as that regex might not work consistently with all of the BNF files. I can keep at that, but wanted to get something initial in before I went too far down the path.
task ui manager was missing an OS import
download manager missing a logging import
removed call to oldDNBLoader from dnbloader as it doesn't exist

[kkdavis14]

@azaroth42
merged your new commits from rob_refactor into this, just to keep it clean/up to date.

my commit removed the edits from base and created a BNF Loader. it's a little complicated, because several of the BNF files store records both in "#about" and without the about. so there's duplicate identifiers, and records split in two. this should work for both cases: when there's just one identifier (the rameau SH file), and when there's duplicates/two records (geos, people & groups).