Add PhfSet
Add PhfOrderedSet

It's a naiive implementation now but can be cleaned up later
Add PhfOrderedSet
Implement ExactSize for relavant iterators
Actually do error checking in parse_set
Use new attribute syntax in README
Update for API changes
Remove unneeded impls
Update for Iterator API changes
Add accessors to underlying keys

@kmcallister requested this so the keys can be used as interned strings.
Update documentation URL
Update for upstream changes
Update travis.yaml to use the nightlies
Update for fmt changes
Update super old syntax in README
Update for format! changes
Use absolute path in expansion

Crates are now required to have the crate accessible as `phf` at the
crate root, which seems better than forcing `use`s.
Fix doc examples
More doc fixes
Update for library changes