WIP: New Sequence data structure

[mortonjt]

Building off of the discussions here

https://github.com/biocore/scikit-bio/pull/1157 https://github.com/biocore/scikit-bio/issues/1159 https://github.com/biocore/scikit-bio-rfcs/pull/9

Here's the tentative TODO list

• [x] port over interval tree from bxpython
• [ ] port over Sequence object from scikit-bio
• [ ] Implement Feature class
• [ ] Implement Interval class
• [ ] implement birectional hashtable to map between intervals and features
• [ ] Overload [ ] indexing method for Sequence object to accept Features as input and return a new Sequence object.
• [ ] Overload [ ] set operation for Sequence object to accept Features as input and set attributes
• [ ] Implement query method to accept an Interval or a **kwargs
• [ ] modify existing reverse complement command in Sequence