Adding 'ipums_usa' method for 1789 to 1930
Update gender_ipums_usa.R

Fixing wording
Merge pull request #10 from bmschmidt/master

Adding 'ipums' method for 1789 to 1930
Tests for IPUMS method

Adds tests for the IPUMS method provided by Benjamin Schmidt in
pull request #10. In addition, if the gender function is not passed
a year or range of years, it now passes an explicit range of years to
the helper functions. The sample names data has been updated with more
helpful names and ranges of years.

Closes #11
Closes #12
Update documentation and bump version

Includes an upgrade to dplyr 0.2