Add test for memoryview of extension type

A test for a bug fixed in commit 478b939a4c99a77c0863cfd.

v2: add commit link above
v3: # tag: instead of # tags:, drop cpp tag as it means something different
    that I originally thought

There was a bug that produced C code where gcc emitted warnings:
extension_type_memoryview.c: In function ‘__pyx_pf_25extension_type_memoryview_test_getitem’:
extension_type_memoryview.c:1468:15: warning: assignment from incompatible pointer type
extension_type_memoryview.c: In function ‘__pyx_pf_25extension_type_memoryview_2test_getitem_typed’:
extension_type_memoryview.c:1565:15: warning: assignment from incompatible pointer type
extension_type_memoryview.c:1568:18: warning: assignment from incompatible pointer type

And g++ failed with errors:
extension_type_memoryview.c: In function ‘PyObject* __pyx_pf_25extension_type_memoryview_test_getitem(PyObject*)’:
extension_type_memoryview.c:1468:213: error: cannot convert ‘__pyx_obj_25extension_type_memoryview_ExtensionType*’ to ‘PyObject*’ in assignment
extension_type_memoryview.c: In function ‘PyObject* __pyx_pf_25extension_type_memoryview_2test_getitem_typed(PyObject*)’:
extension_type_memoryview.c:1565:213: error: cannot convert ‘__pyx_obj_25extension_type_memoryview_ExtensionType*’ to ‘PyObject*’ in assignment
extension_type_memoryview.c:1568:20: error: cannot convert ‘PyObject*’ to ‘__pyx_obj_25extension_type_memoryview_ExtensionType*’ in assignment

--HG--
extra : transplant_source : %02N%D4%B99N%D6%FBv%7C%F0%94%E5%BE%CE%C9t%D6%04%11
removed broken minivect related code from branch since minivect will not be released as part of 0.18
set version to 0.18b1
undo code removal - not entirely clear what to make of it
remove references to 'minierror' module from Shadow.py
move exception class into shadow function as we may not want to export it under the cython.* namespace at this point
merge 0.18.x branch back into master
Added tag 0.18b1 for changeset c1a18ab6b080
improve test output in failure case
suppress C compiler warning on power operation on unsigned C int types
fix test
simplify abs() optimisation for C integers and fix it for the most negative int/long value
avoid unused utility code for abs()
merge 0.18.x branch back into master
optimise abs(long long)
fix compiler crash in error case
merge 0.18.x branch back into master
fix indentation error in userguide
fix indentation error in userguide

--HG--
extra : transplant_source : %7B%A4%F1%C4/%E4l%2C_%BFF%5B%B7%9C%9F%E0_%2B%15%3D
Merge pull request #176 from larsmans/fix-indent

fix indentation error in userguide