Update official build instructions

[Issue migrated from kwant#62 (closed)]

It seems that @basnijholt has had quite some success using conda to build Kwant on OSX. In addition I seem to remember @cwg mentioning some issues with the Golhke Kwant packages for Windows.

Given this, would it make sense to switch the "official Kwant installation instructions" use conda install instead of the current mix of Homebrew (OSX) and Golhke (Windows), that still seems not to work 100% of the time (see kwant#48 (closed))?