civix: Setup license.txt by default

There's been some criticism that new extensions may not include license files unless developers take the effort to copy in the license text. (This parallels this past summer's controversy over licensing on Github projects.)

Through email/IRC, these steps have been identified:

1. Update "civix generate:module" to accept a "--license=X" parameter which will initialize LICENSE.txt and info.xml.
2. Set a default of AGPLv3.
3. Update the output of "civix generate:module" and update the Civi/civix documentation to indicate that this is only a default – that licensing is still intended to be at the discretion of the developer.