Detect parallel emerges

[vincentdephily]

By looking at whether ebuild phases (emerge, Cleaning, Compiling, Merging, AUTOCLEAN...) are interleaved. We could also look at the emerge command line, bit it seems less robust and interesting (running multiple emerge commands, not achieving the max parallelism...).

I don't know if this could be used for predict, but it would be interesting for stats and for human review.

See also issue #49 and issue #53.