Use absolute `cimport` in *.pxd files.  Relative cimporting doesn't appear to work.

Also, alphabetize import in "cytoolz/__init__.pxd" (just because).  The main
reason we don't do `from cytoolz.itertoolz cimport *` is so there is a quick
and easy reference for what is cimport-able.
Remove all instances of `inline`.

Compiling another Cython module using `cytoolz` gave warnings about `inline`.
Issue #5: add `cytoolz.utils.include_dirs` to allow other project to use CyToolz' C API.

A minimal example is included in the docstring, although a basic example
should also be in the hosted documentation (whenever it exists).

This should be regarded as an advanced feature for advanced users, so
typical users should not be encouraged to use it.  This is the main
reason `include_dirs` is tucked away in `cytoolz.utils`.