Add 'spake2/' from commit 'f6e9d07dffa9a9b39f203c23043f93337ebe1ab6'

Add 'spake2/' from commit 'f6e9d07dffa9a9b39f203c23043f93337ebe1ab6'

git-subtree-dir: spake2
git-subtree-mainline: 1afa40d15b0c2cecc989bdfc67a5251aa5cd5954
git-subtree-split: f6e9d07dffa9a9b39f203c23043f93337ebe1ab6

This moves https://github.com/warner/spake2.rs into the "spake2/"
subdirectory of https://github.com/RustCrypto/PAKEs .

I'll move the release tags over next, then update the travis config and
READMEs.