allow user to specify how column types are determined

[wibeasley]

Related to the readr changes that prompted #193, there are several places where REDCapR is throwing warnings, or not converting variables properly. Here are two options that should help

• an guess_type parameter, that forces all columns to be interpreted as strings. This is similar to the as.is parameter for base::read.table().
• a guess_max parameter, that's passed to readr::readr_csv(), which specifies how many rows should be considered when guessing at the data type.