Only load bundler in the rakefile for its gem tasks.

I prefer using binstubs but was getting version mismatch errors from bundler.
Keep qless-lib.lua in source.

This is important so that end users can build lua plugins for Qless and pull in the same version of the qless-lib lua script.
Move lua scripts under a `lua` subdirectory.
Add support for lua plugins.

Fixes #124.
Fix typo.
Read the qless-lib.lua script once, not everytime we create a LuaPlugin instance.
Strip out comment lines when loading lua plugin scripts.
Make Job#note_state_change public.
Add missing `super` call.

Without that, the SHA was not being pre-calculated, which
caused the script to be loaded every time, unnecessarily.
Merge pull request #125 from seomoz/allow_lua_plugins

Allow lua plugins