Use baconjs instead of custom code in hide-message

First real use of external runtime code through commonjs, yay!

It did bloat the compiled script from 144K to 227K, but since it's
a userscript and only downloaded once, it doesn't matter too much.

I thought that baconjs' `assign` function worked differently--it's
actually for method calling e.g. jQuery `attr`, which isn't as useful
for DOM purists.