Updated AbjadAPIGenerator.

AbjadAPIGenerator no longer removes any _static/ directory found during the
docs/ pruning stage of API building.

Why? Because the score manager API was building with a consistent warning.

Here's the old API build output:

    Now writing restructured text files ...

    PRUNING docs/source/_static
    ... done.

    Now building the score manager HTML docs ...

    sphinx-build -b html -d build/doctrees   source build/html
    Running Sphinx v1.2
    WARNING: Abjad no longer supports versions of Python less than 2.7.3!
    WARNING: Please upgrade your version of Python to 2.7.3 or higher!
    loading pickled environment... done
    building [html]: targets for 0 source files that are out of date
    updating environment: 0 added, 1 changed, 0 removed
    reading sources... [100%] editors/index
    looking for now-outdated files... none found
    pickling environment... done
    checking consistency... done
    preparing documents... done
    writing output... [100%] index
    writing additional files... (105 module code pages) _modules/index
    genindex py-modindex search
    copying static files... WARNING: html_static_path entry u'/Users/trevorbaca/Documents/abjad/scoremanager/docs/source/_static' does not exist
    done
    copying extra files... dumping search index... done
    dumping object inventory... done
    build succeeded, 1 warning.

    Build finished. The HTML pages are in build/html.

So this commit changes the AbjadAPIGenerator to leave any _static/ directory in
place. This silences Sphinx's warning when building the score manager API.