set up node test to run Mocha tests
Issue #31: build user inputs matcher on each call to zxcvbn

Prevents crosstalk between different calls to zxcvbn, making it easier
to run as a long lived service (no more memory leak) that is clustered,
etc.

Callers that want the legacy behaviour can accumulate user inputs in an
array of their own.
Merge pull request #51 from atlassian/Make-user-inputs-stateless

Make zxcvbn stateless w.r.t user_inputs