Begin with root key index when reading keystores.

Instead of using the zero value, explicitly set the last chain index
to -1, which represents the root key.  If no additional keys are read
from the io.Reader, this could result in panics when looking up keys
at index 0, when no additional keys have been created yet.

Fixes #119.
IMPORT PATH MODIFICATION CHANGESET
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Please use this changeset as the first one on top of the conformal codebase.
Note that if there are new imports, they need to be changed as part of this
changeset. In general, use "git rebase" on github.com/conformal's version
and apply this changeset first.
Task #396 on Task Board:

Create a stub websocket call getdepositscript.