wooldridge CRAN update: v1.3.0

This update adds six data sets and documentation for the most recent edition of the text,
"Introductory Econometrics: A Modern Approach, 6th edition" (Wooldridge 2016,
ISBN-13: 978-1-305-27010-7). The data sets are: approval, catholic, census2000, countymurders, econmath, meapsingle.

Further compression of data sets by writing a function to delete unnecessary
attributes attached to each data.frame. Excess attributes were assigned to
each data.frame during the import from Stata .dta files. In addition,
the row.names attribute was previously saved as a character type so I converted
to integer, which reduced the size of each data set and the package as a whole.