build: changing behavior to merge develspace

This patch makes the following changes:
 - Makes the default build behavior merge all develspaces
 - Removes the --merge-devel argument
 - Creates a new argument: --isolate-devel = 'not --merge-devel'
 - Updates documentation for this new behavior
Merge pull request #46 from jbohren-forks/merge-devel-default

build: changing behavior to merge develspace