generate-upgrade can now take target_dir argument

Previous generate-upgrade required renames in rel/ directory:

    rebar generate
    ...changing code...
    mv rel/<release> rel/<release_prev>
    rebar generate
    rebar generate-upgrade previous_release=<release_prev>

With the proposed changes you can now do the same without moving things:

    rebar generate # default target dir: rel/<release>
    ...changing code...
    rebar generate target_dir=<release_new>
    rebar generate-upgrade target_dir=<release_new> previous_release=<release>
Print a more appropriate message on 'rebar info'

To avoid treating plugins' info/2 functions as commands, we catch calls
to 'rebar info'. Previously (4b8c81fb) we simply printed the help
string, but it's much more informative to print the following:
Command 'info' not understood or not applicable
Fix #187 (rename getopt to rebar_getopt)

While at it, update getopt.erl to git rev 78eda3b.
Fix #187 (rename mustache to rebar_mustache)
Merge pull request #183 from EchoTeam/generate-upgrade-and-target-dir

generate-upgrade can now take target_dir argument
Merge pull request #196 from tuncer/rebar-run-info

Print a more appropriate message on 'rebar info'
Merge pull request #191 from tuncer/rename-modules

Fix #187 (rename getopt and mustache)