read_cross2: read linemap info
In yaml file, use first character of [Ff]emale and [Mm]ale for sex codes

 - Also, use magrittr; way better than nested function calls

 - Also update developer guide
Add zip_datafiles(), to zip up a set of data files
Small improvement to read_cross2
Add sample data files (zipped)
Revise test-io.R to read local zip files

 - previously was reading sample files from kbroman.org, but was having
   connection problems
read_cross2() can now read data from a zip file
Improved use of magrittr's %>% (thanks @hadley!)
zip_datafiles: fix bug (unintentional global variable)
read_cross2(): able to read zip file from web