Fix for short._ and manual imports.

[stewSquared]

Addresses issue #86 by modifying import short._ to work properly in the scope that the docs suggest. Formerly, the test 'manualImports was defined after an import bundle.all._, allowing errors in the implementation to get through.

Note that the example from the docs that imported:

import bundle.{attrs => attr, styles => css, _}
import bundle.tags._ 

Now requires two additional imports:

import bundle.implicits._
import scalatags.DataConverters._ 

[lihaoyi]

This looks good, thanks!