`DECIMAL` types should be translated to numeric rather than character

[Jarod G.R. Meng]

Currently RPresto converts DECIMAL types to character (see #60 and #64) because the previous translation code (implemented in Rcpp) has limitations. Presto treats DECIMAL type columns like a number (i.e. you can do arithmetic operations on DECIMAL) rather than a string. So it makes sense to translate it into numeric in R. Since the refactoring of the translation code (see #158), this should be quite straightforward.