Speed up multiplying B

We do a lot of multiplication of this constant, so we can
benefit from precomputing its power-of-2 multiples.
Expose a better API than the original DJBware
Use tuple unpacking
Reorgnize as suggsted by @dsufft
py3k fix
Merge pull request #10 from alex/better-exception

Expose a better API than the original DJBware
Speed up multiplying B

We do a lot of multiplication of this constant, so we can
benefit from precomputing its power-of-2 multiples.
Style
Merge branch 'master' into pr/3

Conflicts:
	ed25519.py
Move some comments around
pep8 fixes
Merge branch 'master' into pr/3

Conflicts:
	ed25519.py
Sigh, fix py3k merge conflict mux up
pep8 fixes