Tagging and bagging builds, emscripten to 1.7.1

This adds support for tagging and bagging builds, and bumps emscripten to the 1.7.1 tag.

All builds now include full version and build information in the first few lines of the uncompressed JS file, output to the browser console, e.g.
> MESS BUILD_VERSION == 0.142u6 (Sun Nov  3 09:36:41 UTC 2013)
> EMCC VERSION == emcc (Emscripten GCC-like replacement) 1.7.1 (commit 350550a5ccfacd19f512bca2ded186e143ea9579)
> EMCC_FLAGS ==  -O2 -s DISABLE_EXCEPTION_CATCHING=0 -s ALIASING_FUNCTION_POINTERS=1 -s OUTLINING_LIMIT=20000
> MESS_FLAGS ==   OSD=sdl        NOWERROR=1     NOASM=1        NO_X11=1       NO_DEBUGGER=1  TARGET=mess SUBTARGET=colecovision SYSTEM=colecovision VERBOSE=1   CROSS_BUILD=1 NATIVE_OBJ="/home/milianov/remess/jsmess/mess/obj/sdl/nativemame64" TARGETOS=emscripten PTR64=0 OPTIMIZE=2

Debug builds also include "-debug" in the filename, assuming you uncomment that line in the makefile.

1.7.0 was the first tag after all the fixes emscripten put together for us (detailed in previous pull requests).  A quick test run with Coleco showed no problems.  1.7.1 also showed no problems.