Add support to BigDecimal

This adds support to BigDecimal to this gem. I liked a lot this gem, although I believe that support to BigDecimal values are very important, specially knowing about floating point arithmetic problems (accuracy).

Please let me know if you think this is important for this gem, if it is, also if I need to fix something with this PR, like version (I was definitely not sure about this part).
hm.. i'll think if there is a better way to do that. If not, we will keep it as is.

Just a small remark, could you use `BigDecimal(%s)` method instead of `BigDecimal.new(%s)` constructor here?
Also `benchmark` shows me a better performance of `BigDecimal(%s)`