Easy access to haystack/query residue indices and range

[dwhswenson]

The example notebook previously showed how to find the residues for query/haystack atoms, and then how to get the max/min needed for plotting that subset. But that was more complicated than it needed to be, especially because I'm finding that those "zoomed in" contact maps are something that I want frequently.

This PR adds:

• query_residues / haystack_residues : list of the residue indices for the query/haystack
• query_residue_range / haystack_residue_range : max and min index values of the above

This makes it easy to adjust the plot axes with, e.g.,

ax.set_xlim(*contacts.haystack_residue_range)

Includes tests; updates example notebook.

[sroet]

LGTM, the only thing that had me thinking for a while:
the residue selection in done with

"resSeq 32 to 38"

Which is 1 based while the internal (&plot) index is 0 based

You might want to change to resid for the selection instead, for better matching.
selection ref: http://mdtraj.org/1.6.2/atom_selection.html

[sroet]

LGTM, one final nitpick

[sroet]

LGTM

[dwhswenson]

Made some implementation changes: I decided that these are better as properties, and that haystack_residues/query_residues should return the residues themselves (the mdtraj.core.topology objects) instead of just the indices.

@sroet: I think you have merge access now -- feel free to merge this after appveyor passes!