Updates to sorting interface to better match mainline julia

* Allow user to specify sort order by keyword/column tuple or array
  (e.g., rev=(true,false,false)
* Allow user to specify sort order by order(...) function
  (e.g., order("colname", rev=true) )
* Added issorted(...), and accept the same kw arguments
* Added tests
* Added docs
Merge pull request #389 from kmsquire/sorting_update

DataFrame sorting update