Ability to set full prefix w/o /usr

Greetings,

I was attempting to build some RPM packages and set the fpm --prefix value to /mochi/lib/flex_sdk/

I found out that the prefix definition always appends /usr to the beginning of the prefix option.  I think it would be useful to be able to set the full prefix path starting at root '/'

Worked well with recipe:

```
class Flex46 < FPM::Cookery::Recipe
  description 'Adobe Flex SDK'

  name     'flex_sdk'
  version  '4.6'
  homepage 'http://www.adobe.com/devnet/flex.html'
  source   'http://download.macromedia.com/pub/flex/sdk/flex_sdk_4.6.zip'
  sha256   '622b63f29de44600ff8d4231174a70fcb3085812c0e146a42e91877ca8b46798'
  section 'main'

  depends       'java-1.6.0-openjdk', 'java-1.6.0-openjdk-devel'

  post_install 'post-install'

  def build
  rm_f Dir['runtimes/air/mac/Adobe\ AIR.framework/Adobe\ AIR'], :verbose => true
  rm_f Dir['runtimes/air/mac/Adobe\ AIR.framework/Headers'], :verbose => true
  rm_f Dir['runtimes/air/mac/Adobe\ AIR.framework/Resources'], :verbose => true
  rm_f Dir['runtimes/air/mac/Adobe\ AIR.framework/Versions/Current'], :verbose => true
  rm_f Dir['runtimes/air-captive/mac/Adobe\ AIR.framework/Adobe\ AIR'], :verbose => true
  rm_f Dir['runtimes/air-captive/mac/Adobe\ AIR.framework/Resources'], :verbose => true
  rm_f Dir['runtimes/air-captive/mac/Adobe\ AIR.framework/Versions/Current'], :verbose => true
  end

  def install
  root_prefix("/mochi/lib/#{name}_#{version}").install Dir['*']
  end

end
```

If you think this root_prefix option is a good idea to merge in, I can write a spec test before merge if you want.