Include docstrings in cython code. Fixes API docs. (#76)

setup.py

@@ -32,6 +32,10 @@
     *(glob.glob(os.path.join(MARISA_SOURCE_DIR, path)) for path in MARISA_FILES)
 )
 
+COMPILER_DIRECTIVES = {
+    'embedsignature': True,
+}
+
 DESCRIPTION = __doc__
 # TODO: Switch back to builtin `open(FILE, encoding="utf-8")` when dropping Python 2.7 support
 with open("README.rst", encoding="utf-8") as f1, open(
@@ -98,6 +102,7 @@
                 "src/trie.cpp",
             ],
             include_dirs=[MARISA_INCLUDE_DIR],
+            compiler_directives=COMPILER_DIRECTIVES,
         )
     ],
     python_requires=">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*",