(#1549) - index method for plugins

Adds a new index() API for plugins to access
in order to store additional metadata.

This API was designed with Map/Reduce in mind,
since in order to persist its data it requires
a secondary index not normally exposed by the
adapter API.  However, the new API is
generic enough that it could be useful for
other plugins.
make sure u0000 is replaced
document total_rows
ensure websql destroys plugin data
move plugins to sidebar