Bikeshed: rename “command” to “action”

[cmbrandenburg]

Reasons for the change:

1. The connotations for “action” encompass a broader meaning that is more in line with what a CouchDB client does.
2. The term “action” is gaining broader use with regards to HTTP interfaces. E.g., see Siren.

Although this renaming is technically a breaking change (because the command module will be renamed to action), most practical applications won't break, and most changes to this project will be to the documentation.