Clean RData::Stack implementation

Do not use a separate index to implement a Stack. Ruby's Array
has already all the needed methods.

While there, I've also renamed RData::Stack#is_empty? to RData::Stack#empty?.
It seems to be more in a Ruby style (at least from my point of view).
FIX: Ruby use 2 spaces indent
Merge pull request #11 from bugant/clean-stack-implementation

Clean RData::Stack implementation