Added an additional screenshot utility.

Not only did I add another screenshot utility to print_screenshot(), but I
made it the default. scrot is now the preferred screeshot utility, with
import relegated to a backup option if scrot is not installed.

The ImageMagick import utility works fairly well, but it sometimes results in
black areas of a screenshot when a compositor is in use. Compiz
(used by Unity) and Mutter (the GNOME Shell compositor) don't seem to perturb
it, but Compton (which I use with MATE) and Gala (the Elementary OS/Pantheon
compositor) cause the problems described above, which basically ruins any
screenshot.

I originally used import to take screenshots to maintain compatability with
Stats 1.x. However now that Stats 2.0 has seen some real-world usage it
appears that this bug is too widespread to ignore. Since scrot is another
screenshot utility with a fairly large install base - and, importantly, does
not suffer from this problem - I decided to make it default.
Do not attempt to take a screenshot if the script is running headless.

Up to this commit Stats would try to take a screenshot unless the
--no-screenshot switch was passed. While this behavior makes sense most of the
time, it lead to a bug on headless systems. Stats would try to take a
screenshot if a supported screeshot utility was installed on the system. If
such a utility was available it would fail and print an error message, but
logically this behavior makes no sense.

Since it is fairly trivial to detect whether X is running in the context in
which the script is run, it will now only attempt to take a screenshot if it
is run in a graphical environment. This change invokes the same behavior as
when --no-screenshot is specified.
Added --output-dir option.

This commit introduces a new option to control the output directory for
screenshots and reports. Prior to the addition of this switch the user could
only control the output directory for the files this script generates
implicitly by changing to the desired output directory before executing the
script. The --output-dir switch makes it possible for the user specify the
output directory explicitly without using the aforementioned workaround.

The man page and builtin help information have also both been updated to
document the new switch.
Added Elementary OS detection and logo.

Note: The PNG version of the Elementary OS logo in the logos/ directory is not
quite the same as the original. I generated it from the official SVG, but I
made the borders thicker. No software I tried could properly convert the
original version of the logo to ascii - its was apparently just too thin!
Added a space before the stats printout when --no-logo is in effect.

The statistics and screenshot functions start printing data without first
printing a space. They assumed that if a space was supposed to be printed
before them, it was done outside of the function. This results in everything
being spaced properly (one space between each section) when the distro's logo
is printed because each logo has one space before and after it for balanace.
However when the --no-logo switch was passed to the script, this consistency
was broken because the logo printing function was exclusively responsible for
printing the initial space. As of this commit if no logo is printed spacing
will still be correct.
Stats 2.1.0