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
1.5.6 release
bump version to 1.5.7-SNAPSHOT
merge from master
Merge branch 'develop' of github.com:incanter/incanter into develop
fix calculation of kendalls-tau