Handling string values #52

In parser.jl line 210 it states that "String values do not have labels", but several .sav data files I have worked with contradict that assumption. I have checked that haven can handle it just fine.

The first problem is that the labels are not read into the ReadStatTable, instead just duplicating the string column values. The second (downstream problem) is that we end up with vectors with eltype LabeledValue{String, Union{Char, Int32}} which break _get_string_width() in writestat.jl

I'll raise a PR including a test file (I can't attach an .sav on github) to propose a fix.