attributes: Introduce checksums attribute

Inspired by PR #2, improve the checksum parsing logic and create an
intermediate node[:packer][:checksums] attribute that holds all the
relevant checksums. Then we can simply extract the one we want for the
user by directly looking up the filename.

This remains backwards compatible with any users who are
directly setting :raw_checksums and introduces a new avenue
for those who want to do something different.

Testing done: vagrant provision repeatedly does not download package
Cc: @whilp
attributes: Update for packer 0.3.6
Zap extra whitespace.
README: Describe new checksums parameter

Provide a bunch of documentation to capture intended policy for
raw_checksums (not all checksums over all time for all versions),
and how to select it.
Bump version number for release

There are only re-factorings and backwards compatible changes to
attributes in this release. Also packer-0.3.1 to 0.3.6 does not have any
changes that require "packer fix".