README: added note about --python config setting

Squashed commit of the following:

commit 4b0d2187b86cca9d72ecb5fc1081ebe7a72ae23b
Author: Kevin Ford <kefo@3windmills.com>
Date:   Tue Mar 4 18:20:20 2014 -0500

    Added instructions for selecting specific python version with node-gyp *and* npm.

commit 0995da3a570324fe5fa3fff300a8892d949ec21f
Author: Kevin Ford <kefo@3windmills.com>
Date:   Tue Mar 4 16:05:32 2014 -0500

    Added note about setting npm's Python config key to appropriate Python version.

commit 0d74a8ec9f72c5f0f8f1d437c45b41d2d31d7d23
Author: Kevin Ford <kefo@3windmills.com>
Date:   Tue Mar 4 16:01:31 2014 -0500

    Added note about setting npm's Python config key to appropriate Python version.

commit 8c56de3824dcee5727a66186887fc12a3e3ae533
Author: Kevin Ford <kefo@3windmills.com>
Date:   Tue Mar 4 15:56:20 2014 -0500

    Added note about setting npm's Python config key to appropriate Python version.

commit 5fb6978b3d3c1ce478e2bf513b09a4a04dc8b28c
Author: Kevin Ford <kefo@3windmills.com>
Date:   Tue Mar 4 15:55:19 2014 -0500

    Added note about setting npm's Python config key to appropriate Python version.

Closes #413.