Utilize composer's 'bin' property for global installs.

[jhedstrom]

This change makes use of composer's bin property so that when installing globally, or as part of another project, the binary is properly symlinked into the bin directory (typically vendor/bin).

I wonder if for backward compatibility if we should symlink the binary back into the root directory?

[jhedstrom]

I think the necessary changes have been made here. Anything left to do?