Fetch external libs with composer

I really should have asked about this first, as I'm pretty sure there
are some damn good reasons for doing it the way we're doing :)

Currently, we're including our external libs as they're needed, such as
on the first run or when we try to run a server. It seemed like it would
be more prudent to fetch them when building drush, presumeably before
building for the PEAR repo.

I've stripped out all the utility for sanity-checking all the lib stuff,
and loading it. Everything should be in composer.json now, locked down
just as it was before.

The one thing I needed to do was redefinie the the
`CONSOLE_TABLE_ALIGN_LEFT` constant in environment.inc, since it seems
not to be included when loading PEAR packages via classmap method (it's
defined outside the class).

Anyhow, let me know :)