doc/installation: explain how to verify the integrity of tarballs

I have signed the tarballs at http://code.halmd.org/tar with a trusted
X.509 certificate.
verlet_nvt_hoover: bug fixes, revision of Lua interface

The module was not working at all. Fix several bugs in the Lua interface
(syntax errors, obsolete mandatory arguments, revise file writer, expand
documentation).

The file writer has still an issue (hidden in a comment), which may
point at a bug in luaponte.
add unit test for verlet_nvt_hoover Lua module

The Lua interface of the module has not yet been tested. Similar tests
would be desirable for all modules. They could also simplify the C++
code base of the unit tests (such that the modules are run through Lua
and the results are checked through Boost.UTF).