Dev install instructions

OK, here is an explicit recipe to install Blaze.  It's long but should be copy-pastable and should be robust to Python versions and (I hope) platforms.

I've tested this on 2.7/3.4 on my system.  I expect that it fails on some other configurations.  

@talumbau when you have a chance can you see if this is clear and works for you?
Looks like https://github.com/ContinuumIO/blaze/pull/357 is where this was introduced