Handle multiple modules mixed into another module

[trptcolin]

Looks like my patch in #276 only handled part of the module inclusion issues.

This one handles the case where:

• you have modules A, B, C, and class X
• C includes both A and B, and may also have attributes of its own
• X includes C

Currently, some attributes would get lost depending on what order the includes happened in (see the unit tests).

[coveralls]

Coverage increased (+0.01%) when pulling 6e62b03 on trptcolin:more_module_nesting into 92b5335 on solnic:master.

[tjstankus]

👍

[solnic]

thanks again <3