Merge pull request #28 from github/elr/mod-version

module name must end in v2 for latest go conventions

Merge pull request #27 from github/elr/refactor-to-spdxexp

rename package from expression to spdxexp

Merge pull request #26 from github/elr/refactor-spdxexp

rename spdx package expression

Merge pull request #25 from github/elr/pkg_doc

add package level documentation

Merge pull request #25 from github/elr/pkg_doc

add package level documentation

Merge pull request #20 from github/roman/validate_licenses

Add a function to validate licenses

Merge pull request #19 from github/elr/licenseRef

add support for licenseRefs processing in Satisfies

Merge pull request #14 from github/elr/plus

support for multi-version compatibility for licenses