Expose a function to check whether a command is available in the environment
Make OpamProcess.run a bit more user friendly when you don't care about log files.
Support for opam-<cmd>

when running 'opam foo -x -y', if 'opam-foo' is in the PATH then, OPAM will call 'opam-foo -x -y'.
remark: it's not possible to overwrite default commands.
Update CHANGES