Support FreeBSD 10

FreeBSD 10.0 has "pkgng" of the new default package manager.
pkg_info and such commands has been removed from this version.

I added FreeBSD10 of os type, and tests for "pkgng".
add require freebsd10 into command.rb
Merge pull request #54 from serverspec/support-freebsd-10

Support FreeBSD 10