Add 'p4' (Perforce) as a dependency type

This calls the 'p4' command-line tool to checkout and sync Perforce
trees. It involves significantly more special code in Rebar than
using 'git p4', but it eliminates the indirection of
Rebar->Git->Python->Perforce
Merge pull request #136 from waisbrot/add-p4-support

Add support for the Perforce VCS client via the "p4" tool