Add Composer to correctly install the DI package.
Add support for path injection equivalents for JPATH_SITE,
JPATH_ADMINSTRATOR, JPATH_INSTALLATION and JPATH_MANIFESTS.
Minor bug fixes; remove `index.html` files.
Fix up Composer requirements.
Merge pull request #9 from eddieajau/feature-NamespacedInstaller

More path injection; remove index.html files; convert DI package to Composer distribution model; bug fixes