Allow cppclasses to be declared nogil.

Allow cppclasses to be declared nogil.
Doing so just makes all methods implicitly nogil. This also makes nogil
on a cdef extern from statement do the right thing to cppclasses (and
their methods) contained inside.
---
 Cython/Compiler/Parsing.py  |    3 ++-
 tests/compile/cpp_nogil.h   |    9 +++++++++
 tests/compile/cpp_nogil.pyx |   18 ++++++++++++++++++
 3 files changed, 29 insertions(+), 1 deletion(-)
 create mode 100644 tests/compile/cpp_nogil.h
 create mode 100644 tests/compile/cpp_nogil.pyx