Fixes #215 from GMOD/JBrowse tracker:
Default navigation location can get messed up when ref.start != 0, or ref.end != ref.length

Cherry-picked from 163cf307
Fixes #216, fixes #217:

issue #216: Navigating via sequence dropdown selection ignores
previous location on selected sequence, whereas with navigation text
box entering just a sequence name navigates to previous location on
that sequence (if visited previously).

issue #217: Initial default view of previously unseen seq starts at
80% centered view, but any use of seq pulldown takes it to 100%

Cherry-picked from Gregg's d19bbd8a59d73f3e0dc784b3452fafbe9b65e95c

Conflicts:

	src/JBrowse/Browser.js