Add each_with_index and each_set_index methods

[ggrossman]

Hi @tyler!

I'd like to propose a couple new methods for this gem:

• each_with_index which provides the integer index as well as whether the bit is a 1 or 0. For many use cases, it's useful to know the index of the bit.
• each_set_index which efficiently iterates over only the 1's in the bit set.

Other changes:

• I also got a deprecation warning about rdoctask so I tweaked it to rdoc/task.
• rb_bitset_marshall_load and rb_bitset_marshall_dump are misspelled, should be marshal

Thanks for considering!

[ggrossman]

Ah, now I see #11 by @ericboesch already covers this