srp.js: update docstrings: add 'params' argument, 's' is really 'salt'
remove unused 'salt' argument from server_getS()
pad appropriate return values (v/A/B/S) to the size of the modulus N

Tested by test_picl_vectors for getv, getA, getB, client_getS, and
server_getS.

That test also checks getu, getM, and getK (which are supposed to return
Buffers that equal the size of the hash function). However the test
vectors for those values don't start with a 0x00 byte, so the test isn't
complete. We could use additional vectors to exercise those cases.
pad 'x' to size of the hash, test that u/K/M1 are similarly padded

test_picl_vectors: rewrite, add new vectors to exercise padding bugs.
The new vectors were mechanically generated have leading 0x00 bytes in
x, u, K, and M1.
add input buffer-length precondition checks

We check v, A, and B. We don't check a or b (the secret exponents),
since these are allowed to be shorter than the full modulus. This also
improves the eror message in assertIsNBuffer().
srp.js: join a split line, purely cosmetic
Merge pull request #20 from warner/fixed-length-buffers

make API accept/return fixed length buffers