plotrr 0.3.0

Major changes

Added vignette.

Bug fixes

Incorporated https://github.com/kbroman changes (# 1, @kbroman).
Updated documentation by removing \dontrun{} (# 2).
Added items to .Rbuildignore file (# 3).
Updated title to title case (# 3).
Changed length.unique() to lengthunique() (# 4).