Remove global methods from formulae

Removes any global methods from formulae, and moves #kext_prefix (which seems
to be at least somewhat abstractable) into the Formula class. The only formula
with global methods is now aspell; it (and its generating script in contrib)
has been changed to prefix that method with `aspell_`, to minimize the risk of
name collisions.

Closes mxcl/homebrew#19331.