Add As_shell.{warn,error}.
Fix As_shell verbosity control. Let As_env set the correct value.

Plus a few reorderings in As_shell.
Fix As_shell color control. Let As_env control the value.

The environment variable is ASSEMBLAGE_COLOR, also adds the option
--color.
Support --color and --verbose correctly in setup environment.

Aswell as environment variables.
Logging tweaks.
As_shell.output_exec only redirect stdout.

This makes it easier to diagnose errors when they occur.
Review startup procedure. Better error message if ocamlfind is missing.

And access to help aswell.
Internal checks, warn don't fail.

Failing too strongly can be annoying.
Merge pull request #107 from dbuenzli/samoht-dont-be-fatalistic

Samoht don't be fatalistic