* allow plugins to print help message for implemented commands
* append core rebar.config options to common 'rebar help' message

1. manually insert directory entries to work around issues fixed in
   R15B02 erl_prim_loader
2. put the escript archive's beams in appname/appname/ebin

Tested with R13B03 and R15B01.

By default, executables in paths such as /bin and /usr/local/bin have
the mode saying they can be executed by all.

The current version of escriptize only sets u+x, which creates problems
when copied directly in repositories and requiring other programs to
interact with them.

This change makes rebar follow the standard of linuxes and unixes by
setting the permission flag to a+x, allowing users, the group and others
to execute it.

Allows customizing the arguments handed over to the Erlang VM, e.g. to
set a name or a cookie, e.g. by setting escript_emu_args to:

%%! -name rebar -setcookie rebar

--HG--
extra : rebase_source : 466f440dcb

--HG--
extra : rebase_source : 9eccc596d8

to the OTP bin dir.