Implement const_iterator
Add erase and clear function, have them tested simply.
Complete copy constructor and assignment operator, add find_prefix
function
Add get_key() in iterator,
add trie_set
refine erase() to return an iterator after the element removed.
Have further test: using user-define type and key and value in trie_map
remove key_elem in trie_node, which results better memory efficiency.
some new tests
test of travis-ci
ignore bin
untrack bin
delete bin\
test travis-ci again
travis again
travis again
travis again
10 'travis again's