Rename toxenv

[ryanhiebert]

We use the name toxenv in several places, because the TOXENV environment variable is the mechanism I used to implement the primary feature of Env Detection. When #78 is merged, it will no longer be used, except to make sure we don't do anything if it's already been given. So let's not use it as a name.

Some places that I know it's used:

• The name of the module that implements env detection. (Rename to detect or envlist)
• The name of the function that implements env detection. (Name the same as the module above)
• The name of the file with the documentation for env detection. (Name the same as those above)
• The name of the setting for the [travis:after] section that says which envs to look for. (Either the same as the above, or perhaps just env. Keep the current name for backward compatibility.)