Adding 'ipums' method for 1789 to 1930

This adds a new method, 'ipums', that classifies based on first names in the US census.

It's based on the IPUMS samples of the following censuses, at the rates listed:

It allows classification before the ssa set begins in 1880, and given [problems with the SSA set](https://github.com/ropensci/gender/issues/9) may be better for years between 1880 and 1915 or so.

The code used to derive this from the raw data is (mostly) available [here](http://rpubs.com/benmschmidt/gender)

Data citation:
Steven Ruggles, J. Trent Alexander, Katie Genadek, Ronald Goeken, Matthew B. Schroeder, and Matthew Sobek. Integrated Public Use Microdata Series: Version 5.0 [Machine-readable database]. Minneapolis, MN: Minnesota Population Center [producer and distributor], 2010.



```
Sample	Density	Note
1850 1%	1.0%	
1860 1%	1.0%	
1870 1%	1.0%	
1880 10%	10.0%	
1900 5%	5.0%	
1920 1%	1.0%	
1930 5%	5.0%	
1940 1%	1.0%
```