require srp.verifier to be 512 characters

 pad the verifier generated by client/index.js
temporary fix for hanging verification tests
Merge pull request #152 from dannycoates/i133

temporary fix for hanging verification tests
return a 400 error for incorrect verification codes
respond with an error on incorrect verification codes
Merge pull request #147 from dannycoates/i147

SRP Verifier should be padded to 256 bytes for N = 2048
Merge pull request #153 from zaach/incorrect_code_error

Error response for incorrect verification code
Adding password stretching