Change library to use dynamic API of capnproto.

Currently only get is working on DynamicStructReader, but
the mechanics are mostly there.

capnp_cpp.pxd

@@ -0,0 +1,26 @@
+# schema.capnp.cpp.pyx
+# distutils: language = c++
+# distutils: extra_compile_args = --std=c++11
+# distutils: libraries = capnp
+from schema_cpp cimport Node
+
+cdef extern from "capnp/dynamic.h" namespace "::capnp":
+    cdef cppclass DynamicValue:
+        cppclass Reader:
+            pass
+    cdef cppclass DynamicStruct:
+        cppclass Reader:
+            DynamicValue.Reader get(char *) except +ValueError
+            bint has(char *)
+
+cdef extern from "capnp/schema.h" namespace "::capnp":
+    cdef cppclass Schema:
+        Node.Reader getProto()
+        StructSchema asStruct()
+    cdef cppclass StructSchema(Schema):
+        pass
+
+cdef extern from "capnp/schema-loader.h" namespace "::capnp":
+    cdef cppclass SchemaLoader:
+        SchemaLoader()
+        Schema load(Node.Reader &) except +