Deprecate Base.boot keywords :company and :version_copyright

Replace them with :author and :attribution, respectively.
Reject nil supplied values for required Base.boot keywords
Merge pull request #4 from njonsson/base-boot-keywords

Refactor and validate Base.boot keywords
Fix deprecations resulting from commit b114fff5a70293482940f4c2e427d49254376a00.
Fix README as a result of commit b114fff5a70293482940f4c2e427d49254376a00.
Bump version.