Fix severe issue with long file paths on Mac OS X

Fixes a bug introduced in commit 183d3ba. On Mac OS X, k2pdfopt
shortens long file paths for some reason. The effect of this was
that k2pdfopt generated a file with an unexpected filename if
the requested file path was too long. We work around this by
(1) using a shorter filename for the (temporary) output file of
k2pdfopt and (2) we run k2pdfopt from the directory of the
output file so that we can refer to it by relative path and
don't need to include the whole path in the command line
argument. Measure (1) does not affect the final name of the
file after the transfer/email step but it does affect the meta-
data that is included in the PDF document.
Pragmatic fix to issue with pref-window resizing on Mac

Fixes #16.

In fact, this just makes the initial size of the iframe slightly
bigger. An actual dynamic fix seems difficult (as indicated by
reports of frustration with simmilar issues on stackexchange).
It's related with browser.preferences.animateFadeIn being set to
true on mac. This flag seems to generate a lot of problems,
also for other projects.
Minor UI tweak on welcome page.

Hide the input field for the verification code if it
turns out that the e-mail address had already been
verified before.