This change allows to specify a number of directories in addition to the system and user DUB package directories to search for local packages.

Full list of changes:

 - Add support for the DUBPATH environment variable (a (semi-)colon separated list of paths)
 - Add support for "dub add-path" and "dub remove-path" to add static search paths
 - The branch of local packages with no "version" field in their package.json is now inferred using "git rev-parse"
 - The structure of the user/system package directories is changed from "packages/<packname>/<packver>" to "packages/<packname>-<packver>"
 - There is no project local package directory (".dub/packages") anymore - packages are instead installed to the user package directory by default
 - Upgrading and uninstallation is partially broken in this commit - will be fixed later