select app version based on CPU features

[davea]

Add new fields to app_version: 1) a regular expression matched against the "features" list (SSE etc.)

(note: this assumes that features are listed in a consistent order; is this the case? Is there a better way?)

2) a regular expression matched against the "accelerators" list

(GPUs and other coprocessors)

3) a "value"

The scheduler would send the highest-value app version that matches the host.