Update docco to latest version

The latest Docco now uses Highlight.js instead of Pygments, which
nicely avoids an inconvenient system dependency.
Require pg version 1.0.0 or higher
Update API for compatibility with pg 1.0+

As of version 1.0.0, pg deprecates the methods `Client#pauseDrain()`
and `Client#resumeDrain()`. Clients are now returned to the pool by
calling `done()`; a new third parameter provided to any callback given
to `pg.connect( url, callback(err, client, done) )`.

In response, our API now accepts the `done()` function provided by
`pg.connect()` as the last parameter when constructing a new
Transaction instance. When the root transaction is finally committed
or rolled back, we now take care of calling `done()`.
Accept variable arguments in constructor
Update README for new pooling API
Add docs for pooled client transactions
Remove the non-pooled interface in API docs
Update examples for the new API
Add History file
Version 0.2.0