Added content_types method that gets an array of content types for a file
Added mime_type methods and filter_by_content_type
Return empty array when mdls returns nothing
Replaced check for OS X with existing maid function
Inspect paths in commands in case of special characters
Add attribution for @JohnColvin
Fix spec on Linux

  1) Maid::Tools#downloaded_from should determine the download site
     Failure/Error: @maid.downloaded_from('foo.zip').should == ['http://www.site.com/foo.zip', 'http://www.site.com/']
       expected: ["http://www.site.com/foo.zip", "http://www.site.com/"]
            got: [] (using ==)
     # ./spec/lib/maid/tools_spec.rb:315:in `block (3 levels) in <module:Maid>'