feat(slugbuilder): allow build without using a pipe

This is backwards-compatible with existing deis/builder. This allows
the extracted app to be mounted at /mnt/app. Ref deis/deis#785

The pipe was causing some issues with Python's popen implementation.
See this [teardown] for more information.

Some legacy code that used curl was removed.

[teardown]: https://github.com/deis/deis/issues/785#issuecomment-52605987