Add support for "restore --sparse"

Thanks to Patrick Rouleau <prouleau72@gmail.com> for spotting a
serious potential short-write problem in an earlier revision of this
work, and suggesting other improvements.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>
[rlb@defaultvalue.org: v5 fix bup_write_sparsely docstring; use "m"
 instead of "M" for dd counts (for OS X)]
Add .travis.yml

Signed-off-by: jmoore <josh@glencoesoftware.com>
Add travis-ci build status PNG

Signed-off-by: jmoore <josh@glencoesoftware.com>
Fix travis-ci dependencies

Signed-off-by: Rob Browning <rlb@defaultvalue.org>