improve behaviour of `rebar shell`

attempt to emulate the behavior of
`erl -pa ebin -pa deps/*/ebin`

fix error messages and formatting issues of `rebar shell` by
shutting down and restarting the user subsystem in a mode more
hospitable to the shell than the simple user started when run
as an escript. emulate `error_logger` behaviour when the shell
is run via `erl`

add documentation of the shell command

limitations:

the erlang interrupt handler is not enabled when running as an
escript and there is no interface to re-enable it via erlang code.
this means `ctrl-c` will immediately exit the running process
unlike when running the shell via `erl`. `ctrl-g` is, however,
unaffected

the user subsystem is killed and restarted but not supervised. if
your code somehow relies on the user subsystem crashing and
restarting `rebar shell` may interfere with it's operation
Merge pull request #280 from talentdeficit/bettershell

improve output when using `rebar shell`