Update licensing and dependency statements

[swinslow]

As discussed in #134, there are a handful of new dependencies with the support for new file formats. @swinslow to update the descriptions of dependencies and corresponding licenses.

In particular, note that gopkg.in/yaml.v2 is Apache-2.0, so the YAML file license expressions should likely be updated to Apache-2.0 OR GPL-3.0-or-later -- since Apache-2.0 is compatible with GPL-3.0 but viewed by some as incompatible with GPL-2.0.