Implement basic non-wget download methods

Packages can now be sourced from Git and Subversion repositories. The
download method will be autodetected from the URI (git://, svn://, etc).
If the repository is accessed through http(s), you can force the
download method by setting a _SITE_METHOD variable to either 'git' or
'svn', respectively and without the quotes.

The package's _VERSION variable defines which commit, revision, tag or
branch should be checked out. For Git, it can be HEAD, a commit ID, a
tag name or branch name (anything that can be checked out with `git
checkout`). For Subversion, it must be a revision number, or HEAD.

Signed-off-by: Maxime Petazzoni <maxime.petazzoni@bulix.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>