Add alias support to formulae

* brew install will find an aliased formula
* aliases are searched against
* warn when creating a new formula that has an existing alias.

If Subversion has an alias "svn", then warn when the user tries to
create a new formula "svn". The formula can still be created, though
the user should make sure it's not a duplicate of the existing
aliased one.

Subversion and Objective-Caml formulas get some alises here, so we have
something to test against.