Details:
- Modified .travis.yml to automatically employ the simulation of
application-level threading within the testsuite, with supporting
changes to common.mk, the top-level Makefile, and
travis/do_testsuite.sh.
- Added a new pair of input files to testsuite directory with the
'.salt' suffix (similar to those with the '.fast' suffix) for
testing application-level threading.
- Updated docs/BuildSystem.md to document the new make targets
'testblis-salt' and 'checkblis-salt'.
