update phf::PhfMap to phf::Map

phf public interface updated in commit:
sfackler/rust-phf@7fc934a23e7e25fd12014a123eea8f7707928338
update generator usage doc to reflect phf::PhfMap -> phf::Map change
Merge pull request #12 from DanielFerguson/fix-phfmap-naming

update phf::PhfMap to phf::Map
(fix) Define RawMediaType in content_type for static linking.

By moving the definition, we can prevent linking against generator
and allow static linking. This is kind of a hack, but it works and
this is a simple crate, so that is ok.

Fixes #13