Makefile: introducing latexmk, wildcards, continuous build mode, ...

using `latexmk` to produce the PDF, wildcards to list the files, `clean` & `distclean` with `latexmk` (which parses the log files), `show` target, `cont` a continuous build mode (very handy), changing `export` directory to `proposal-export` and `dist` tags the PDF with the GIT revision.