Small fixes in molfiles-module

I slighlty adapted the branch molfiles-module (kindly contributed
by T. Giorgino).

* Moved LICENCE file to COPYRIGHT
* Run ./header.sh script to have consistency with other plumed headers
* Made search for external plugins the default, internal ones are used
  as fallback solution
* Made it possible to disable molfile completely (./configure --disable-molfile-plugins)
  This could be necessary if also the internal plugins do not compile correctly
  (we might remove this possibility in the future)
* Re-generated autoconf with 2.69
Modified changelog