Make info message clearer
Improve build process

Two main changes:
* automatically fetch BWA, modify its Makefile, build its static library
* turn the library produced by rapi_bwa into a thin archive merging the symbols
  in libbwa.a, so that client builds do not need to reference libbwa.a when linking
Adapt .travis.yml to modified build procedure
Have swig print its version in the build log