resolve #261, explain args to callback to #del

Per bug report, example crafted from test.
better document #del method

Thanks for the feedback, @rvagg.
Based on your feedback and some double checking via
https://github.com/rvagg/node-leveldown/blob/5d552d750170bedd0656d2e6d82673556fa0ea0d/deps/leveldb/leveldb-1.17.0/db/db_impl.cc#L1437-L1441

I've gone with a comment similar to the preceding stanza.
Also removed the curly braces.
Use newer memdown store. Fixes build.
explicit devdep versions
support values to be null/undefined