Add LGPL 3.0.

Locket will be released under LGPL 3.0. All the supporting libraries
will also be released under LGPL 3.0. At the time of this commit, no one
is using Locket and I am the sole copyright holder, so this license
applies not only to this version but to all prior versions of Locket,
regardless of the license files that are captured by the version control
software used to manage the editing of this project.

Closes #91.