luaexpatutils: New package

lxp.doc is a module that provides useful features for working with XML
formats in LOM format as used by the LuaExpat project from Kepler.

[Peter: use fixed git revision, github tarball, install -D]
Signed-off-by: Assaf Inbal <shmuelzon@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>