Minor fixing and improvements of options

This pull request fix/add the following:

* Add -v option for show program version.

Example:

<pre>
  $ gifme -v
  gifme 0.0.8
</pre>

* Add -q option for silence program output

* Fix bug with zero arguments that produces nil:NilClass (NoMethodError)

Before to fixing it displays:

<pre>
$ gifme
/var/lib/gems/1.9.1/gems/gifme-0.0.8/bin/gifme:65:in `<top (required)>': undefined method `[]' for nil:NilClass (NoMethodError)
	from /usr/local/bin/gifme:23:in `load'
	from /usr/local/bin/gifme:23:in `<main>'
</pre>

Nows shows the usage and help.

* Improve usage message displaying the options

The current ouput is like:

<pre>
$ gifme -h
Usage: gifme [-h] [-r] [-o /path/to/output] [-d DELAY] [-w PIXELS] [-q] [-v] FILES [FILES ...]

  FILES can be listed out, like `file1.jpg file2.jpg`, or it
  can be a normal shell glob, like `*.jpg`.

Options:
    -r, --reverse                    Reverse the GIF to make it loopable
    -o, --output /path/to/output     Set the animation's output directory
    -d, --delay DELAY                Set the delay between frames (default: 20)
    -w, --width PIXELS               Set the width of the image (default is 500px)
    -v, --version                    Shows the program version
    -q, --quiet                      Don't print status messages to stdout
    -h, --help                       Show this message
</pre>

Happy gifming!