initial attempt at performance testing suite for notmuch.

[david at tethera.net]

For a long time we have needed some commone set of tests to measure
changes to e.g. tagging and dump/restore. A non-trivial corpus of mail
messages is a bit unwieldy to be shipped with the source, so there is
a seperate tarball.  Currently the tarball for this needs to manually
uploaded to the notmuchmail.org http server; I have set up the first
version.