Allow a job requirement for Not Equals and Not Exists

Add in a "not" component to specifying agent requirements in a job.

Example:
"I want an agent that I can compile 32 bit builds on, but is not a 64bit machine. It must be explicitly a 32 bit operating system"

supports32bit exists
supports64bit does not exists

Example:
"I want an agent that isn't a CentOS 5.3 machine, but is fine for every other version of CentOS"

Operating_System does not equal CentOS 5.3 (this would allow for agents of CentOS 5.5, 6.0, etc)