add lone period to nastrings array

Federal Reserve St. Louis has a rich set of databases and for some reason they use a single "." to denote missing data. 

Currently:

````julia
               DATE  VALUE
[1,]     2013-10-14    "."
[2,]     2013-10-15 "2.75"
[3,]     2013-10-16 "2.69"
[4,]     2013-10-17 "2.61"
[5,]     2013-10-18 "2.60"
[6,]     2013-10-21 "2.63"
[7,]     2013-10-22 "2.54"
[8,]     2013-10-23 "2.51"
[9,]     2013-10-24 "2.53"
[10,]    2013-10-25 "2.53"
[11,]    2013-10-28 "2.54"
[12,]    2013-10-29 "2.53"
````

Pull Request result:

````Julia
               DATE VALUE
[1,]     2013-10-14    NA
[2,]     2013-10-15  2.75
[3,]     2013-10-16  2.69
[4,]     2013-10-17  2.61
[5,]     2013-10-18   2.6
[6,]     2013-10-21  2.63
[7,]     2013-10-22  2.54
[8,]     2013-10-23  2.51
[9,]     2013-10-24  2.53
[10,]    2013-10-25  2.53
[11,]    2013-10-28  2.54
[12,]    2013-10-29  2.53

````