Add "bup index --clear" to clear the index.

Signed-off-by: Zoran Zaric <zz@zoranzaric.de>
Reviewed-by: Rob Browning <rlb@defaultvalue.org>
bloom-cmd: don't explicitly create the bloom file (allow bloom.create()).

Don't explicitly create the bloom file since bloom.create() will
handle it, and will actually use the correct (binary) mode instead of
"w+".

The binary mode has no effect under Linux or Cygwin, but is important
under Windows.

Signed-off-by: Patrick Rouleau <prouleau72@gmail.com>
Reviewed-by: Zoran Zaric <zz@zoranzaric.de>
Reviewed-by: Rob Browning <rlb@defaultvalue.org>