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
Style
Merge branch 'master' into pr/3

Conflicts:
	ed25519.py
Move some comments around
pep8 fixes
Fix xrange for Python 3

scalarmult_B is hot enough that it's probably best to stick with
xrange when in Python 2, so let's make 'xrange' work in Python 3.