rearrange test_picl_vectors.js: no functional changes

Just renaming things to make it easier to tell which are inputs and
which are outputs. I did remove the P->P_hex encoding test, since we
don't actually use the raw "P" password anywhere (we only use
P_stretched).
further test reorganization: same vectors

This removes tests of non-SRP things like encoding decimal numbers as
integers and back. It removes all mention of the unstretched password.
It switches to using a Buffer (created from a hex string) everywhere
possible.
update PICL test vectors to current version

These should match the vectors from:

 https://wiki.mozilla.org/Identity/AttachedServices/KeyServerProtocol#Test_Vectors

as of 26-Aug-2013. That means srpK = e68fd0112bfa31dc ffc8e9c96a1cbadb
4c3145978ff35c73 e5bf8d30bbc7499a.
Add srp.getM(), the key-confirmation message.

Closes #13.
Merge pull request #18 from warner/getM

add 'getM' (and update test vectors in the process)