Generate retention field for dataports.

If both values are 'infinity' won't display them.
Validate or create retention
Passes old spec_test
Retention tests that pass.
Merge pull request #34 from tadpol/addingRetentionToSpec

Adding retention to spec command
fix exoline tree --values example output
Merge branch 'master' of github.com:exosite/exoline
alignment
Update README.md

add note about how to upgrade exoline using pip.