Added mixed-datatype testing to Travis CI (#271).

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'.

travis/do_testsuite.sh

@@ -8,9 +8,10 @@ export BLIS_JC_NT=1
 export BLIS_IR_NT=1
 export BLIS_JR_NT=1
 
-if [ "$TEST" = "FAST" ]
-then
+if [ "$TEST" = "FAST" ]; then
     make testblis-fast
+elif [ "$TEST" = "MD" ]; then
+	make testblis-md
 else
     make testblis
 fi