proposal, move nmbug from ./contrib to ./devel

[David Bremner]

I mentioned this again on IRC today, and nobody seemed too upset by the
idea. I propose to move nmbug from the contrib subdirectory to the
devel subdirectory.   It seems relatively integral to the notmuch
development process at this point, and at least two of the regular
contributors are helping maintain it.

I'm not posting this as a patch, because file renames are not very
interesting patches.

d