add RackTables extensions ver. 0.3.1

This is the contents of racktables_extensions_v0.3.1.tar.gz w/o the
.svn/ directories and with some fixes to files permissions.
rearrange files on filesystem

Move the PHP files to plugins/ and update them to use addJS() and
addCSS().

Move the non-PHP files to directories under plugins/ that can be served
by the RackTables static content proxy and fix URIs respectively.
update README and version information

RackTables 0.19.x is not supported any more; with 0.20.x and the files
laid out properly the installation becomes notably simpler.
add cable ID helper plugin version 0.2