Makefile, introducing latexmk and wildcards

* all: using latexmk to produce the PDF
* clean: for removing tmp files
* distclean: to get rid of everything
* show: build pdf and open it
* cont: continuous build mode, very handy
* export: directory is proposal-export now
.gitignore: an additional latex tmp file
wrong target for make cont in makefile
Merge pull request #4 from haraldschilly/master

Merged it on my own. Is there any formal process in place?