add lifetables data from SSA
add live births data from census.
ignore the .xls file
Merge pull request #4 from ramnathv/master

add lifetables data from SSA
Convert to tidyr
More births data
Tidy life tables code
Minor data tweaks
Add url
Re-roxygenise
Add cran comments
Merge branch 'master' of https://github.com/hadley/babynames