add dev-db/mongodb

TODO: system sm and snappy don't work
dev-db/mongodb bump to 2.0.5
dev-db/mongodb: fix deps
scons: dev-db/mongodb: use arbor's scons.exlib
dev-db/mongodb bump to 2.0.6
dev-db/mongodb: don't rename download
mongodb: bump to 2.4.3

Signed-off-by: Marc-Antoine Perennou <Marc-Antoine@Perennou.com>
mongodb: simplify tests, always run default
mongodb: bump to 2.4.7

Signed-off-by: Marc-Antoine Perennou <keruspe@exherbo.org>