[WIP] Extension installer

Supports PHP & HHVM.

Todo:
- [ ] Need to come up with a solution for ext-* packages actually being installable now, so extensions can be compiled automatically to satisfy dependencies on extensions.
- [ ] Documentation
- [ ] Use a different downloader to download binary builds from github releases for windows (or pecl for PHP)
- [ ] Output information on configuring the server to include extension.so

Replaces the earlier PR #498