Rakefile: add target to download SJCL
Rakefile: add target to download SJCL

In order to have the JavaScript built properly, we need components from
SJCL.  Add a target to download these components and verify them against
a known-good hash, and to extract them in the right place.

Signed-off-by: brian m. carlson <738bdd359be778fee9f0fc4e2934ad72f436ceda@crustytoothpaste.net>

bk2204/daniel-ruby,bk2204/daniel-ruby