Remove all unit tests from the processcompat module as many of them failed (not an issue since the new std.process is now in DMD 2.063).
Add unit tests to the path module and make \\UNC\ paths absolute.
Compile fix for -inline compilation.
Add exception messages.
Add support for DUBPATH and "add-path" + change the default installation location to the DUB user path. Fixes #64.

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