Issue title: Linux - Unsupported system command: "mdls -raw -name kMDItemDurationSeconds
 Issue body: Hi - just tried getting started with maid on linux/64bit/12.04

after installing it - icreated a sample rule - edited it - and continued with the test-function from your instructions. While running

> > maid --dry-run

with  maid-0.1.0.gem i get

Rule: MP3s likely to be music
/var/lib/gems/1.8/gems/maid-0.1.0/lib/maid/maid.rb:85:in `cmd': Unsupported system command: "mdls -raw -name kMDItemDurationSeconds \"/home/fidel/Downloads/SOME_MP3_FILE.mp3\"" (ArgumentError)
    from /var/lib/gems/1.8/gems/maid-0.1.0/lib/maid/tools.rb:89:in`duration_s'
    from ./.maid/rules.sample.rb:20
    from ./.maid/rules.sample.rb:19:in `each'
    from ./.maid/rules.sample.rb:19
    from /var/lib/gems/1.8/gems/maid-0.1.0/lib/maid/rule.rb:4:in`call'
    from /var/lib/gems/1.8/gems/maid-0.1.0/lib/maid/rule.rb:4:in `follow'
    from /var/lib/gems/1.8/gems/maid-0.1.0/lib/maid/maid.rb:74:in`follow_rules'
    from /var/lib/gems/1.8/gems/maid-0.1.0/lib/maid/maid.rb:72:in `each'
    from /var/lib/gems/1.8/gems/maid-0.1.0/lib/maid/maid.rb:72:in`follow_rules'
    from /var/lib/gems/1.8/gems/maid-0.1.0/lib/maid/maid.rb:47:in `clean'
    from /var/lib/gems/1.8/gems/maid-0.1.0/lib/maid/app.rb:23:in`clean'
    from /var/lib/gems/1.8/gems/thor-0.14.6/lib/thor/task.rb:22:in `send'
    from /var/lib/gems/1.8/gems/thor-0.14.6/lib/thor/task.rb:22:in`run'
    from /var/lib/gems/1.8/gems/thor-0.14.6/lib/thor/invocation.rb:118:in `invoke_task'
    from /var/lib/gems/1.8/gems/thor-0.14.6/lib/thor.rb:263:in`dispatch'
    from /var/lib/gems/1.8/gems/thor-0.14.6/lib/thor/base.rb:389:in `start'
    from /var/lib/gems/1.8/gems/maid-0.1.0/bin/maid:5
    from /usr/local/bin/maid:19:in`load'
    from /usr/local/bin/maid:19

am i guessing right that this is some osx-specific setting?
If so - is there an option to disable the osx-related AND not in linux supported parts to avoid errors?
Or am i totally wrong and the error is somewhere else?

Please tell me if you are missing some relevant informations

Best regards in advance.

 Comments: 
 Comment 0: Hi @macfidelity,

Thanks for posting this issue; I'm happy you find Maid interesting.  The issue you mentioned is something I'd definitely like to improve, as I'm gradually switching from using Mac OS X to using Ubuntu.

The methods that are specific to OS X are listed [in the documentation](http://rubydoc.info/gems/maid/0.1.0/Maid/Tools).  They are tagged with "Mac OS X".  For the time being, it would be best to avoid them.

There's [some code to detect whether the host OS supports a given command](https://github.com/benjaminoakes/maid/blob/master/lib/maid/maid.rb#L92), but clearly it's less than foolproof.  As a longer term strategy, I'd like to have feature parity between OS X and Ubuntu (if possible).

I'm open to alternatives, as well of course.  :smile:

Best wishes,

Ben

 Comment 1: Hi,

i guess i just thought that the sample rules coming from using "maid sample" are expected to work on linux. Thanks for pointing me to the documentation - that answers the error ;)

Suggestions:
- on a short-time-view it might help to add comments to the example-rules generated using "maid sample" to point out which os/platform is supported.

greetings

 Comment 2: I like that idea.  I might do a point release with just that.

 Comment 3: I've released Maid v0.1.1 with the suggestion you mentioned, plus other various minor changes.

 Comment 4: Hi,

thanks for considering the idea.
Lets hope it helps someone else who might try to get started with maid ;)

 Comment 5: :)

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