o New project configuration controlling how the sandbox behaves
when `bst shell` is used; allowing projects to provide a more
functional shell environment.
o The `bst shell` command now has a `--mount` option allowing
users to mount files and directories into the sandbox for
testing purposes.
o Log lines are now configurable with the new "message-format"
user configuration, allowing one to express optional fields
such as microsecond precision and wallclock time.
o Newly added filter element
o Git source plugin now allows disabling of submodule checkouts
o In the same way we allow overriding element configurations
by their 'kind' in project.conf, we now support the same
for source plugin configurations.
o Tar and zip sources now automatically recall an `etag`
from the http headers, optimizing tracking of tarballs
significantly (issue #62)
