Add simple exponential backoff to approve transfer wait

[jrwdunham]

Fixes #114

This should allow us to wait 17.5 hours for the approve transfer select to appear, where before we only had 8.3 minutes:

>>> s = 1
>>> m = 64
>>> for x in range(990):
...   t += s
...   s = 2 * s
...   if s > m:
...     s = m
>>> (t / 60.0) / 60.0
17.510833333333334

Testing required to confirm fix.

[jrwdunham]

Closing in favour of #119