function for retrieving the header file path (fixes #158)

pybind11/__init__.py

 from ._version import version_info, __version__
 from ._version import version_info, __version__
+
+
+def get_include():
+    import os
+    try:
+        from pip import locations
+        return os.path.dirname(
+            locations.distutils_scheme('pybind11')['headers'])
+    except ImportError:
+        return 'include'