Postpone importing numpy until compiling starts

[jkawamoto]

This lazy evaluation enables CVXcanon to be pip installed even if numpy isn't
separately installed beforehand, because pip is able to interpret the
requirements from setup.py. Previously, since the setup file for CVXcanon began
by importing numpy, pip would error if numpy wasn't already installed.

For example, suppose your requirements.txt is like

numpy>=1.12.0             
scipy>=0.18.1             
CVXcanon>=0.1.1

and run pip install -r requirements.txt to install those packages,
pip checks dependencies written in install_requires in setup.py in order to
install those dependencies before compiling CVXcanon starts.
However, since setup.py imports numpy at the top, this checking step fails due
to an import error.

This pull request introduces a lazy import of numpy to postpone importing
it until actual compiling phase starts, and it allows pip to evaluate setup.py
and install dependencies first.

After numpy and scipy are installed, the compiling CVXcanon phase starts.
pip now evaluates the lazy import to obtain the include path of numpy.
At the time, since numpy has been installed, no import error raises.

You will also no longer need pip install numpy as a first step in the CVXcanon
installation instructions, if missing it will be installed when CVXcanon is pip
installed.

[SteveDiamond]

Excellent catch! Thanks!