Sequence::rev_comp works on subsequences

Sequence::rev_comp works on subsequences
ticket:197
Sequence::rev_comp when called on a Sequence which contained a subsequence
was actually reversing the entire sequence and not just the selected region.

To fix this I added rbegin and rend which return reverse iterators that
cover the subseq range and updated rev_comp to use them.

There's also some extra sequence unittests to make sure that rbegin/rend
work correctly. (and as a bonus loading several sequences out of one fasta
file works).