Fixed some deprecation warnings
Fixed 'opam switch' when OPAMSWITCH is set

running 'opam switch' once you had run 'eval $(opam config env --switch) could get very confusing
Better display truncate
Protect Sys.signal calls
New parallel processing engine

Don't fork opam anymore, put parallelism at the external command calls level.
OpamProcess.Jobs offers an interface for parallelisable jobs containing external
command calls. The OpamParallel module now deals with such jobs by running
commands concurrently in the background and waiting on child processes when needed.

Limitations yet:
* downloads and everything but package build/install has been unparallelised
* verbose mode is'nt very nice
* output still a bit hackish
Rewriting calls to use the new parallel engine (backends other than http disabled)
Reimplement all backends for the new process architecture

local, darcs, hg should now be OK
Fixed location of log files
parallel engine: better dl reporting
log: pid no longer useful
Parallelise dev package update
Update: better display
Remove the 'packages UPDATED' messages unless in verbose mode

they're not that useful since we get the update summary, have been proved confusing
on occasions and don't carry much worthwile information
verbose mode: display command output at end, don't call tee
Be more verbose on compiler download and install
Downloads: for consistency, display message when done
Fixed error reporting on build
Fixed 'removed' message at failed install
Better reporting of downloads
Refactored action processing

Now using a graph of atomic actions (removals, installs) for better
granularity and causality analysis