AppVeyor tests of compilation in Windows environments

[smmaurer]

This PR adds an AppVeyor script to test compilation in Windows environments.

It looks like some modifications would be required to make our full test suite run in AppVeyor, so I've skipped that for now. Some earlier work on that is archived under the tag appveyor-work-2017.

Results

I confirmed that compilation fails in Windows Python 2.7 prior to PR #123, but succeeds after including those changes:

• https://ci.appveyor.com/project/smmaurer/pandana/builds/27157927/job/xt3t2hrbbfixvaif
• https://ci.appveyor.com/project/smmaurer/pandana/builds/27159795/job/adffd0her3n4fpb4

Documentation

This PR also updates the documentation a bit:

• clarifies that cython and numpy must be installed prior to running setup.py
• advises of problems using Pip to install runtime requirements in Windows Python 2.7
• additional general cleanup of installation instructions

[coveralls]

Coverage remained the same at 91.667% when pulling 7765c79f1bc87123a7e164248ca82fcf5f0a8bb0 on appveyor-compilation into a723ab488b9d16b150f62c742c7f933dba08e4f2 on develop.