mu-sexp-convert: Fix conversion of elisp times.

Previously, the first element was used where the second should have been.

In essence, whereas the algorithm is T[0]<<16 & T[1], the code was previously performing T[0]<<16 & T[0].

The timestamp produced was therefore inaccurate by up to about 18 hours.
Merge pull request #147 from mmirate/patch-1

* mu-sexp-convert: Fix conversion of elisp times.
* mu4e: update doc for mu4e-create-maildir-maybe
* mu4e: don't try to select nil window when updating mail