explicitly check for named and mutating attributes on transformations
add drop_percent_missing column_transformation
Update select_rows.r
fix description
update docs
Merge pull request #6 from robertzk/drop_percent_missing

add drop_percent_missing column_transformation
add weak param for select_variables
Merge branch 'master' of github.com:robertzk/mungebitsTransformations
weak fillls in NAs