Make it possible for plug in alternative protocol buffer compilers.
The compilers are picked up based on if they export all of the
functions key/0, proto_compile/3, proto_clean/3 and proto_info/2.
The set of compiler modules to choose from, is fetched from the rebar
application environment, from the app_dir modules.
A new config option, {proto_compiler,Compiler}, specifies which of
the available protocol buffer compilers to use. The 'protobuffs'
compiler is now one such compiler (the only one), and it is also the
default, for backwards compatibility.
