Exposing SchemaParser in Cython header

[cjermain]

This PR exposes the C_SchemaParser thisptr to any external Cython library that cimports from capnp.lib.capnp. While this is not intended for general use, the availability of the C SchemaParser in the Cython header is critical for loading a compiled type and dependencies. With this feature, external Cython libraries can load the compiled type, and then cast the DynamicStruct::Reader to the appropriate reader of that type. The following code serves as an example for taking advantage of this method.

In a Cython file:

from capnp.lib.capnp cimport SchemaParser, C_SchemaParser, _DynamicStructReader, C_DynamicStruct
import capnp.lib.capnp as capnplib

cdef extern from *:
    """ // The following code is evaluated as C

    void load_type(capnp::SchemaParser &parser) {
        parser.loadCompiledTypeAndDependencies<AddressBook>();
    }

    AddressBook::Reader cast_reader(capnp::DynamicStruct::Reader &reader) {
        return reader.as<AddressBook>();
    }
    """
    void load_type(C_SchemaParser parser)
    AddressBook.Reader cast_reader(C_DynamicStruct.Reader reader)


def initiate_schema():
    """ This should be called once to load the appropriate type,
    before using cast_to_compiled_type.
    """
    if capnplib._global_schema_parser is None:
        capnplib._global_schema_parser = SchemaParser() # Singleton

    load_type(capnplib._global_schema_parser.thisptr) # <<<<< thisptr provided by this PR

def cast_to_compiled_type(_DynamicStructReader reader):
    """ Takes a Dynamic Reader and casts it as a specific compiled type.
    """
    cdef AddressBook.Reader address_reader = cast_reader(reader.thisptr)

[cjermain]

This build fails because Python 3.3 support was dropped from setuptools (and pkg_resources). Two solutions are:

1. Drop support for Python 3.3, since it has reached its end-of-life.
2. Fix the setuptools version in travis.yml to one prior to version 40.

@jparyani, any thoughts? I'm happy to put up a separate PR for either.