Make sage -i http://.../foo.spkg work immediately

[Jeroen Demeyer]

The following works right after extracting the Sage source tarball:

./sage -i /path/to/foo.spkg

But this doesn't work for remote packages:

./sage -i http://example.com/foo.spkg

or

./sage -i foo

Trac #13373 implements this using Wget, curl or system Python. Needs
review: http://trac.sagemath.org/sage_trac/ticket/13373