Fix problem with metadata for ncol & nrow functions

fixes #169
re-implement to-vect as polymorphic function, similar to to-list

this fixes #164
sel now returns dataset even if result contains one row, and we selecting the columns

this fixes #165
refactor the code, close file even if the exception is thrown
1.5.2 release
bump version to 1.5.3-SNAPSHOT
Add set-point-size for scatter plots
Add tests for set-point-size and adjust formatting
Use Clojure's "group-by" in $group-by for speed

Change $group-by so that it uses the builtin group-by function and add
tests for $group-by. Now substantially faster for some datasets.
Test commit for Travis CI
Merge pull request #173 from morrifeldman/master

Use Clojure's "group-by" in $group-by for speed
Add rename-cols

Rename columns by name or number
Merge pull request #174 from morrifeldman/rename-cols

Add rename-cols
fix link to Incanter's group