Details:
- Modified .travis.yml to automatically test the mixed-datatype support
of the gemm operation, 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
'.mixed' suffix (similar to those with the '.fast' suffix) for testing
mixed-datatype gemm.
- Updated docs/BuildSystem.md to document the new make targets
'testblis-md' and 'checkblis-md'.
