change add_concept to add_entry

This should have been done in the previous release, but I missed it.
Change add_concept to add_entry to reflect the change from conceptEntry
to entry in the TBX-Min dialect.
pretty-print XML

Use XML::Twig to pretty print the return value of as_xml instead
of XML::Writer's format.
as_xml returns ref

The documentation for as_xml says it returns a scalar ref,
so change it to actually return a ref (instead of a plain scalar).
fix add_entry doc

The documentation incorrectly specified that the method takes a
TBX::Min::LangGrp object, which is incorrect.
Updated TODO
unused variable

Fix failing release test which complained that $el was only used once.
New CPAN release of TBX-Min - v0.05

    Fixed warning on 5.10.X
    add_concept changed to add_entry as should have been done in 0.04
    as_xml returns pretty-printed XML
    as_xml return scalar ref instead of just scalar