ENH: linalg: Add the `axis` keyword to linalg.norm.

Also fixed a bug that occurred with integer arrays and negative ord.  For example,
norm([1, 3], -1) returned 1.0, but the correct value is 0.75.
MAINT: linalg: removed unused imports from linalg.py
DOC: linalg: Improve the docstring of the new norm function.
ENH: linalg: allow the 'axis' argument of linalg.norm to be a 2-tuple, in which case matrix norms of the collection of 2-D matrices are computed.
STY: linalg: some PEP8 clean up.
MAINT: linalg: simplify the computation of the svd-based matrix norms, and tweak the norm function's docstring.
Merge pull request #3387 from WarrenWeckesser/norm-axis

ENH: linalg: Add an `axis` argument to linalg.norm