Basic move semantic for ptree

[maciekgajewski]

Added move semantic for the container: move constructor and move assignment,
and also move versions of the following methods:

• push_back
• push_front
• insert

In the future, I'd also like to add move versions of the put/add family.

[nvkhoi112358]

Any reason that this is not accepted to master?
Also, I think the add_child should support move semantic too.
For big ptree, it cost a lot on copying.

Current I have to add/put first, then modify on returned reference.