Better recode.na function

The `recode.na` function is now more cautious with numeric strings.

Here's the new output for each example provided in the documentation.

    > require(questionr)
    > source("/Users/fr/Documents/Code/R/questionr/R/recode.r")
    > data(hdv2003)

With exact string matches:

    > freq(recode.na(hdv2003$nivetud, "Inconnu"))
    Recoded 0 values to NA.
                                                                      n    %
    N'a jamais fait d'etudes                                         39  2.0
    A arrete ses etudes, avant la derniere annee d'etudes primaires  86  4.3
    Derniere annee d'etudes primaires                               341 17.1
    1er cycle                                                       204 10.2
    2eme cycle                                                      183  9.2
    Enseignement technique ou professionnel court                   463 23.2
    Enseignement technique ou professionnel long                    131  6.6
    Enseignement superieur y compris technique superieur            441 22.1
    NA                                                              112  5.6

With regular expressions:

    > freq(recode.na(hdv2003$relig, "[A|a]ppartenance", "Rejet|NSP"))
    Recoded 1292 values to NA.
                              n    %
    Pratiquant regulier     266 13.3
    Pratiquant occasionnel  442 22.1
    NA                     1292 64.6

Showing missing values:

    > freq(recode.na(hdv2003$clso, "Ne sait pas", verbose = TRUE))
    Recoded 27 values to NA.
                 n
    Ne sait pas 27
           n    %
    Oui  936 46.8
    Non 1037 51.8
    NA    27  1.4

Test results with `freq` (as shown above too):

    > freq(recode.na(hdv2003$trav.satisf, "Equilibre"))
    Recoded 451 values to NA.
                      n    %
    Satisfaction    480 24.0
    Insatisfaction  117  5.9
    NA             1403 70.2

Truncate a count variable (recommends numeric conversion):

    > freq(recode.na(hdv2003$freres.soeurs, 5:22))
    Recoded 505 values to NA.
    Recoded variable contains only numeric characters. Consider using as.numeric = TRUE.
         n    %
    0  167  8.3
    1  407 20.3
    2  427 21.3
    3  284 14.2
    4  210 10.5
    NA 505 25.2

I'm also coding a `freq.na` function to inspect the proportion of missing values in a vector or any number of variables.