Bug 906918 - Use MoveRef's in nsTHashtable. r=bsmedberg

This patch adds a move constructor for nsTHashtable and also lets
nsTHashtable use its EntryType's move constructor, if one is available.

This patch also adds move constructors for nsCharPtrHashKey and
nsUnicharPtrHashKey.  Using these new constructors, nsTHashtable will be
able to avoid strdup()'ing all of its keys whenever it resizes the
table.