Various make file changes

Changes to common/Makefile:
* Remove 'dist' target ( as vimperator-labs at googlecode.com is no more)

* Use command line arguments that exist both in the GNU and BSD flavour of 'cp' and 'sed' commands so that build works out of the box on Linux, Mac and Windows (using Cygwin)

* Remove unknown files before building the xpi

* Print out the correct location of the built xpi file at the end of the build process