Module management commands and unloading enforcement

This pull request implements the following commands:
- MODULES
- LOADMODULE
- UNLOADMODULE
- RELOADMODULE

It also ensures that any non-core modules that work with modes will unset said modes when they are unloaded. However, to make this work I had to make a slight change in the order of `unloadModule` in `IRCd`. It used to remove the mode chars from the dictionaries before calling `fullUnload`. This wouldn't let modules unset their modes, as txircd was confused into thinking the mode chars they were trying to unset did not exist.
If you're using `sorted` here, you're not really using the iterator, which makes the choice of `iterkeys` misleading.  `keys` is better here.