'show' output now tab-delimited.

Changed from comma-delimited. This saves the extra 'tr' step when
sending data into gnuplot.
Updated 'help'.
'show' and 'dump' accept magic doc ids.

'first', 'last', numeric index now allowed as benchmark ids. You can now
do 'esbench show first' to show the first benchmark only.
Merge branch 'an' into dev
Bugfix to 'client' parser.
Added 'num_search_segments' to default fields.
Updated bench to work with multiple shards.

Segment stats are now correctly aggregated in Observation._segments()
when dealing with an index which has multiple shards, primary [and]
secondary.

In addition, ability to specify the number of shards (primaries) at
command line with the '--shards' argument.

Ability to record detailed per-segment information removed (no need for
that kind of detail, leads to needlessly complex observation data).

Version bump to 0.2.1