Add missing source code file from pip package.

The actual source code is missing in the git repository.
I downloaded the 0.1.0-release from pip and added the missing file.
Fix bug in screenshot() on linux and mac

screenshot() fails because Image.open() is lazy and does not load
the actual data from the file before it's deleted. Fix by explicitly
calling its load() method.
Fix bug in locateAllOnScreen on linux/mac

On these platforms, screenshotIm *has* an fp attribute, but it's set
to None. Cover all options by catching AttributeError.
Fix bug in scrot detection on Linux

The problem with the original subprocess.Popen call was that with
shell=True the whole command, including arguments, need to go in the
first string, and subsequent strings are fed as arguments to /bin/sh,
which always results in returncode 1.

Since shell=True is not needed anyway, this problem can be solved by
removing it.

Then there's a very hypothetical deadlock problem with the use of the
wait() method, and just to be safe I fix that too by using the
solution discussed here: http://stackoverflow.com/a/5195443/566644

Finally change the except clause to only catch the expected error and
let all other errors propagate, to avoid the error hiding antipattern.