Added PLMD::molfile namespace

Now all the types and functions declared in the
local version of molfile plugins are properly wrapped
in a namespace.

Possibly fixes #110
Renamed some global function

added "plumed_" prefix to some global functions.
Hopefully fixes #110

I would like @tonigi to have a look at it before merging!
Revert "Renamed some global function"

This reverts commit 69728b725778a2511ca4570b66ea816dba07aca2.
molfile plugins should now be only in their PLMD::molfile namespace
(no C linkage).
voodoo attempt to remove extern from stuff which is not extern
Merge pull request #111 from tonigi/fix-110-tg

No "extern C" in molfile plugins
Merge branch 'fix-110' into v2.1

internal molfile plugins are now contained in PLMD namespace