add CYTHON_CACHE_DIR env

allows orthogonal data to be put in the cache dir.

default inline location unchanged (~.cython/inline),
but get_cython_dir() is .cython, not .cython/inline.

resolution for the Cython cache dir:

1. CYTHON_CACHE_DIR env if defined
2. platform cache dir
   - (OS X): ~/Library/Caches/Cython if ~/Library/Caches exists
   - (posix not OS X): XDG_CACHE_HOME/cython if XDG_CACHE_HOME defined
3. ~/.cython/inline

Fix -Wreserved-user-defined-literal error in C++11.

    error: invalid suffix on literal; C++11 requires a space between literal and identifier
          [-Wreserved-user-defined-literal]
          "Item size of buffer (%"PY_FORMAT_SIZE_T"d byte%s) does not match size of '%s' (%"PY_FORMAT_SIZE_T"d byte%s)",
                                  ^

Went to town, sorry if I got carried away.

Mark or anyone - I've left a few comments in the ReST for places where I
could not make the particular feature work, or where I wasn't sure on
naming.

tests pass when compiled and run manually.

fixes typo in memory view validation code