Truncate before write in mock file system

Fixes #81
Adds missing mock method to fs-root.js
Update change log
Port all specs to Jasminum
Rename specs to tests.

Because they are tests. Not specifications.
Use method, not function, when calling reroot recursively
Simplify behavior of reroot

Don't implicitly reroot to the base of the prefix path.
Version 2.0.0

Not for the faint of heart.