Add cairocffi.xcb

Create cairocffi.xcb module, which uses xcffib as a library to create
XCBSurface objects
Add xcb information to mkconstants

Add cairo-xcb.h functions to constants as
cairocffi.constants._CAIRO_XCB_HEADERS
Add xcb install option

Installs xcffib for xcb support when running:

    pip install cairocffi[xcb]
Add xcb documentation

Add documentation for cairocffi.xcb module to sphinx docs
Add xcb tests

Add test cases for the new cairocffi.xcb module. Two test cases are for
creating XCB surfaces on pixmap and window.