major cleanup of the tutorial chapter on string handling to match the future behaviour and new features of Cython 0.13

Previously it didn't work if followed step by step (one has to link the
Rectangle.cpp file). This patch makes everything clear and explicit.

--HG--
rename : index.rst => src/userguide/index.rst

--HG--
rename : docs/early_binding_for_speed.rst => src/userguide/early_binding_for_speed.rst
rename : docs/extension_types.rst => src/userguide/extension_types.rst
rename : docs/external_C_code.rst => src/userguide/external_C_code.rst
rename : index.rst => src/userguide/index.rst
rename : docs/language_basics.rst => src/userguide/language_basics.rst
rename : docs/limitations.rst => src/userguide/limitations.rst
rename : docs/numpy_tutorial.rst => src/userguide/numpy_tutorial.rst
rename : docs/overview.rst => src/userguide/overview.rst
rename : docs/profiling_tutorial.rst => src/userguide/profiling_tutorial.rst
rename : docs/pxd_package.rst => src/userguide/pxd_package.rst
rename : docs/pyrex_differences.rst => src/userguide/pyrex_differences.rst
rename : docs/sharing_declarations.rst => src/userguide/sharing_declarations.rst
rename : docs/source_files_and_compilation.rst => src/userguide/source_files_and_compilation.rst
rename : docs/special_methods.rst => src/userguide/special_methods.rst
rename : docs/tutorial.rst => src/userguide/tutorial.rst
rename : docs/wrapping_CPlusPlus.rst => src/userguide/wrapping_CPlusPlus.rst