Fail quietly for incompatible PHP syntax files

Fixes #5.  The Hack syntax file is written to augment the default PHP
syntax file that ships with Vim 7.4.  This note has been added to the
README, and the Hack syntax file now defines all syntax groups it
references before using them so that it doesn't scream too loudly if
some other syntax file is used.
Highlight type, newtype, shape, and this

Fixes #6.  Note that indenting for shape type declarations is broken.
This will not be fixed because it can create ordering conflicts with the
vim-xhp bundle.  #dealwithit