Update to latest crow, resolve link errors.

This resolves compatibility issues on the (as of writing) latest version of crow, and resolves linker errors related to the kTypeNames and kSelf variables on Apple LLVM 6.0/clang 3.5 (OS X 10.10.1, Xcode 6.1.1).

This also resolves the TODO of disabling the 'Server' header by setting it to an empty string in after_handle.

Figured I'd make a PR here, this seems like a nice little middleware for Crow.  I hate to see it fall behind!