Allow users to make new trees (fixes 056bc12e4cd8)

Commit 056bc12e4cd8 added support for readonly trees by looking at the
"can_edit" property. When a new tree is added it doesn't have a set of
permissions from the API so "can_edit" was essentially returning nil.

Now we only prevent editing if the API explicitly returns a permission
set.
Merge pull request #12 from ahinz/bug/dont-overapply-readonly

Allow users to make new trees (fixes 056bc12e4cd8)