MAINT: Remove doc/numpybook.

Remove doc/numpybook. It has become outdated in parts and its contents
is now part of the official numpy documentation.
DOC: Document removal of doc/numpybook in 1.9.0 release notes.
Need to INCREF dtype again since PyArray_NewFromDescr steals a reference
BUG: Fix commutativity of {dot,multiply,inner}(scalar, matrix_of_objs)

Fixes #2469 and #4482. Relaxes fail condition to allow subtype as documented.
Merge pull request #4488 from charris/remove-numpybook

MAINT: Remove doc/numpybook.
DOC: Change pyrex to Cython in c-info.how-to-extend.rst.
Remove excessive spacing
Merge pull request #4493 from charris/reference-cython-not-pyrex

DOC: Change pyrex to Cython in c-info.how-to-extend.rst.
doc fixes to 'array-protocol type strings'

The item size is not always used (and not always means exact bytes), and
'O' was not in the table.
explain character codes of dtype.kind

"one of 'biufcSUV'" is not very helpful if it stands alone,
also the 'O' typecode was missing.
"(byte-)string" in PyArrayInterface.typekind docs
ENH: release gil for indexing operations

closes gh-4310
ENH: release gil for put and putmask
DOC: specify that 2d array must be non-empty in np.poly error message.

Fixes #2092. Changes message passed to ValueError.
Merge pull request #4067 from hmeine/master

explain character codes of dtype.kind
Merge pull request #4498 from meltingwax/meltingwax/2092

DOC: specify that 2d array must be non-empty in np.poly error message.
Merge pull request #4497 from juliantaylor/index-gil-release

ENH: release gil for indexing operations
MANT: rename variable to avoid shadowing

The variable `obj` in `prepare_index` is shadowed in an inner scope within
the function. Renamed the inner scope variable to `tmp_obj` to avoid
confusion.
DOC: suggest definition of unique symbol in .h

I find it much more convenient to define the PY_ARRAY_UNIQUE_SYMBOL in a
header file #included by all files of the extension (than to repeat its definition in each and every file).
Merge pull request #4500 from jaimefrio/mapping-shadowing

MANT: rename variable to avoid shadowing