Sel returns dataset when :rows :all specified

Before the fix a simple list was returned for datasets with a single row
when :cols :all :rows :all was used. Although sel is supposed to return
simple lists only when at least one of the query params :rows and :cols
is an atomic object (index or name).

Fixes github issue #235
Merge pull request #259 from nblumoe/fix/sel-on-dataset-with-single-row

sel returns dataset when :rows :all specified
bugfix for #261
fixed test to handle new rank-index function
Merge pull request #262 from etosch/master

bugfix for #261
update median to return Double/NaN when provided an empty list
Merge pull request #263 from chausler/master

Update median to return Double/NaN when provided an empty list
JC: Add sel filter test
JC: Add group-on test
JC: Follow sel :filter rename to group-on
Merge pull request #267 from johnchapin/group_on_fix

Follow sel :filter rename to group-on
update "Same as R's ..." to "Equivalent to R's ..." for greppable consistency
Add instructions how to include incanter in project.clj and usage example.
- Added functions to manipulate labels (has-series? & remove-series)
- Added a function to extend existing line based on label (extend-line)
Added a test for extend-line and the other series functions.
Allow save-svg to write to a stream
Merge pull request #279 from cayennes/svg-stream

Allow save-svg to write to a stream
Merge pull request #277 from nbeloglazov/documentation

Improve README
Merge pull request #278 from malloc82/extend-series

Added some plot functions to extend existing series.
Merge pull request #270 from moquist/upstream

update "Same as R's" to "Equivalent to R's" for greppable consistency