IOS fix slow contact access due to photos temp file generation

"Do-while test if tmp file exists" costs a lot. Use mktemp instead to generate photo tmp file.

Before (300 contacts with pictures):
5minutes

After:
10 seconds


NB: I made also a pull-request on phonegap central repository : https://github.com/phonegap/phonegap/pull/19
I figured out after that this repo would be the right place.