Add :platform options to collect_data

@sersut @danielsdeleo @adamedx @lamont-granquist @nordsieck-oc 

Making a PR not because this is finished, but because I want to leave you with some notes.

The following spec tests are converted and guaranteed to work:
spec/ohai/dsl/plugin_spec.rb
spec/unit/system_spec.rb
spec/unit/loader_spec.rb
spec/unit/runner_spec.rb

None of the plugin spec tests are guaranteed to run. I have not tried a full Ohai run with pry/irb. Some plugins may be broken, but I believe most are functioning.

Some major plugin changes/files to look at are:
hostname
kernel
ps
uptime

along with platform-specific plugins
platform
virtualization

Aix and Hpux have been absorbed by Sigar. Some *bsd platforms have common plugin files. Those have been combined appropriately in either Freebsd (for all *bsd platform) or Netbsd (for Net- and Openbsd only).

I created a 'common' library/module located in the ohai/common directory. There are some plugins that need to be split into separate plugin files (e.g., virtualization) but still share a significant amount of common code. You'll find in ohai/common/virtualization.rb that common virtualization code in an Ohai::Common::Virtualization module. Check out the platform-specific virtualization plugins (linux, *bsd, solaris2) to see what that looks like.

Apologies for my commit log. It is immature and not particularly well structured.