NA in levels vs NA in values

[hadley]

This is an attempt to take a comprehensive look at handling NAs in levels vs NAs in values:

• A new terminology that leads to fct_na_levels_to_values() and fct_na_values_to_levels() (and deprecating fct_explicit_na())
• A brief introduction to the issue in the forcats vignette (Fixes #326)
• Tests and some small changes to ensure that you can use other_level = NA everywhere. (Fixes #291)