bugfix: solve multi authorize bug

It appears that if currentSession is called multiple times, let's say we have

call 1
call 2
call 3
call 4

it resolves like this

resolve 2 : null
resolve 3 : null
resolve 4 : null
resolve 1 : session

So we keep the promise in a variable to execute only once.

I think we can do cleaner. What do you think @balessan @clement ?

Closes #34 (closed)