This commit adds a PainlessExtension which may be plugged in via SPI to
add additional classes, methods and members to the painless whitelist on
a per context basis. An example plugin adding and using a whitelist is
also added.
This commit adds the infrastructure to plugin building and loading to
allow one plugin to extend another. That is, one plugin may extend
another by the "parent" plugin allowing itself to be extended through
java SPI. When all plugins extending a plugin are finished loading, the
"parent" plugin has a callback (through the ExtensiblePlugin interface)
allowing it to reload SPI.
This commit also adds an example plugin which uses as-yet implemented
extensibility (adding to the painless whitelist).
Ryan Ernst