BLD/CI: cython cache pxd files
Currently the cython cache on travis doesn't pick up change in `.pxd` files  Most of this commit history is trial and error - but 479c311 shows this working

https://travis-ci.org/pydata/pandas/jobs/166041112

```
$ ci/prep_cython_cache.sh
cython_files.tar  motd.legal-displayed  pip  pyxfiles
Cache available - checking pyx diff
util.pxd has changed:
--- /home/travis/build/pydata/pandas/pandas/src/util.pxd 2016-10-08 13:01:48.255250369 +0000 +++ /home/travis/.cache/pyxfiles/home/travis/build/pydata/pandas/pandas/src/util.pxd 2016-10-06 11:04:00.000000000 +0000 @@ -97,5 +97,6 @@ cdef inline bint _checknan(object val): return not cnp.PyArray_Check(val) and val != val + cdef inline bint is_period_object(object val): return getattr(val, '_typ', '_typ') == 'period'
In a PR
Rebuilding cythonized files
Use cache (Blank if not set) = true
Clear cache (1=YES) = 1
```

xref #14359
