reorder-columns

Added a new function `reorder-columns` for a dataset that will change the order of appearance of the datset columns.  It does not alter the row order.

This should help with the conj-rows issue in #78
add "tail" function
fix typo
support negative lengths
Merge pull request #140 from farrellm/tail

Fix for issue #139
update dependencies, use fresh clojure version
Merge pull request #133 from davidjameshumphreys/align-data-cols

reorder-columns
rewrite reorder-columns more idiomatic way, using the sel
add-* functions should return chart object

this should fix #141