modified new_from_element() to ignore non-blessed items

The change was needed because Web::Query methods assume $self->{trees} items are objects, but HTML::TreeBuilder sometimes (one case in t/data/html5_snippet.html) includes a bugus ' ' item in the list returned by $tree->guts().
Merge pull request #17 from tokuhirom/improved_new_from_element

modified new_from_element() to ignore non-blessed items