core: check for Ruby 2.0
core: use keyword args to simplify BoxCollection#add
core: support checksum on box add
core: output UI when checksumming
kernel/v2: support checksum on box config
website/docs: document box checksum flags
website/docs: document checksumming from Vagrantfile
core: do proper type conversions for box checksum types
core: require file checksum for box add
Revert "core: check for Ruby 2.0"

This reverts commit 9be6b4f0d3fb12ba4d85b54b5afc19f9d7a4c772.
Revert "core: use keyword args to simplify BoxCollection#add"

This reverts commit 4281af338f7d2d1a75c102233fab0e699eaab3a2.