Just in time variable

For those variables that are rarely needed and therefore inappropriate for cluttering up the bootstrap-loaded variables.

Originally inspired by the User Variable D6 project. I deliberately chose to develop something more limited in scope in order to keep the interface very compact and, possibly, to make integration easier with modules that use the current core variables system.

A D7 version of User Variable could conceivably be built on top of this project by using multipart name construction, rather than DB columns, to identify a desired variable.

Latest: Now complete (I think) in terms of mimicking the core variable handling. jit_variable_delete() was the last thing to go in.