Fix documentation of mahalanobis-distance

`mahalanobis-distance` returns a sequence of 1x1 matrices. The examples given expect a sequence of numbers, though.
Adding add-column, replace-column, add-derived-column functions.

These give us more ways of adding columns to datasets.
io/read-dataset now converts empty fields => nil, or user-supplied value.

Also, refactoring of the function for clarity.
Remove trailing whitespace
Swap order of candle-stick-plot docstring and args

Additionally, update default chart title to "Candle Stick Plot".
Update default titles in docstrings for charts/plots

Updated docstrings:
* time-series-plot
* scatter-plot
* line-chart
* bar-chart
* pie-chart
* box-plot
* function-plot
Merge pull request #184 from mneilsen/master

Minor updates to charts.clj documentation
add melt function

this fixes #177. Thanks @aaelony for initial version
Merge pull request #182 from michaelochurch/missing-data

io/read-dataset now converts empty fields => nil, or user-supplied value...
Merge pull request #162 from smee/patch-1

Fix documentation of incanter.stats/mahalanobis-distance
bugfixes for dataset?, pow, atan2 functions

this fixes #183
Merge branch 'master' of github.com:liebke/incanter
Merge pull request #178 from michaelochurch/dataset-add-column

Adding add-column, replace-column, add-derived-column functions.
1.5.3 release
Merge branch 'master' of github.com:liebke/incanter
1.5.4 release
bump version to 1.5.5-SNAPSHOT
refactor scripts

all test/install/clean/push scripts are now using the helper script `run-cmd` that
iterates over modules directories and executes given command.  It can be used for
execution of any commands, like `lein marg` in every directory, etc.

This fixes #186
fix documentation for melt function
catch divide by zero exceptions when calculating rho-k

following the scipy implementation,
https://github.com/scipy/scipy/blob/master/scipy/optimize/optimize.py#L8
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