COMP: Set graduated testing options

BRAINSTools_MAX_TEST_LEVEL adjusts how agressive the test suite is
so that long running tests or incomplete tests can easily be
silenced
1 - Run the absolute minimum very fast tests (These should always pass before any code commit)
3 - Run fast tests on continous builds (These need immediate attention if they begin to fail)
5 - Run moderate nightly tests (These need immediate attention if they begin to fail)
7 - Run long running extensive test that are a burden to normal development (perhaps test 1x per week)
8 - Run tests that fail due to incomplete test building, these are good ideas for test that we don't have time to make robust)
9 - Run silly tests that don't have much untility
set(BRAINSTools_MAX_TEST_LEVEL 3 CACHE STRING "Testing level for managing test burden")
Update README

Added Markdown headings and information about testing levels