Add relx support with active app option

On project init, concrete now prompts for active app (y/n). If you
answer yes, then the generated project is an active application with a
minimal relx.config. Running `make rel` will produce a release in _rel/
Remove description from project generator

Querying for the description was more annoying than useful and gets in
the way of getting someone started quickly with a working skeleton.

Also removing license and copyright headers from generated files.
Use local rebar if present

With this change, we use local rebar if present, fall back to PATH, and
finally fall back to install.
concrete init now creates the directory for you

The new command line is: concrete init DIR

This further streamlines the interaction since the DIR arg is the
project name. So concrete only has to ask if you want an active
application.
Use a packaged rebar for concrete template commands
Improve command execution handling

We'll now give useful debug output if commands fail.
Merge branch 'sf/relx'