recode.na: better handling of numeric conversions

the function returns factor, recommends to convert "1" "2" "3.4" to
numeric, stays silent otherwise, and accepts as.numeric as an option
allow spaces in numeric detection
freq.na (a table to count missing data)

Basically a wrapper around table(is.na(x)) that works for both data
frames and vector objects.
adding GSS example data
indents in happy.Rd
GSS citation
indentation
Merge pull request #4 from briatte/master

Functions for missing values, and adding another example dataset