middleware/token: store the token in current ctx

[bajtos]

A follow-up PR for #337.

See also the discussion in #569

/to @raymondfeng please review
/cc @ritch

[bajtos]

@raymondfeng one of the older tests are failing, because loopback.getCurrentContext is not defined. I fixed that by providing a dummy loopback.getCurrentContext returning null.

I have also change the real implementation to return null when loopback.getCurrentContext is called from outside of a request-handling chain, e.g.

app.use(loopback.token()); // this calls getCurrentContext
app.use(loopback.context()): // the context is started here
app.use(loopback.rest());

LGTY?

[bajtos]

@raymondfeng ping

[raymondfeng]

LGTM