Ignore the content of the download directory
Remove doc target from make file
Various Makefile changes

        * Remove 'dist' target

        * Use command line aruments that exist both in the
          GNU and BSD flavour of 'cp' and 'sed' commands

        * Remove unknown files before building the xpi

        * Print out the correct location of the built xpi
          file at the end of the build
Merge pull request #58 from zklinger/makefile-fix

Various make file changes