nci: add default key bindings file
nci: update palette handing.

Each buffer class can now define it's own palette
clean up __init__ imports
nci: fix document file opening when file not available or found
nci: add a copyID method to grab document id
nci: clean up imports.
trivial code cleanup
nci: fix up default buffer status string.
nci: improve default search key bindings, and add copyID
nci: fix help pages.
nci: minor code cleanup
doc: use list_sources to get list of sources.
update TODO
update default keybindings file, which still needs to be integrated.
tweak readme
fix some typos.
debian: add missing dependency on pybtex
sources: remove references to ads and dcc

which are unfortunately currently not actually supported.
update/fix up README
Provide appropriate error message for nci if python-urwid not installed.