[#1124] Add which function support. Fix createNSIS method to work on Windows systems.

Problem description
===============

We are unable to automatically build the Server distribution on Windows machine at this time.

We would like to be able to do this regardless of the operating system.


Changes description
===============

Moved `which` method from Server's `pavement.py` to `brink/path.py`. Added `extra_paths` parameter to be searched if command is not found in the system path(s).

Changed `createNSIS` method to use `which` for determining the location of `makensis` executable file and render it OS agnostic.

How to try and test the changes
=======================

Unfortunately neither lint nor test targets seem to exist for brink. Please advise if I need to 
add/configure this functionality or we will revert to source-code review only.

*Thanks for your help!*

reviewers @adiroiban