Add a FromIterator wrapper to push a table

[tomaka]

Allow pushing a whole table at once by building it from an iterator.

[mkpankov]

I'm not sure how FromIterator would work, as it needs to create table in Lua end and it needs reference to interpreter for that. However, its' signature is

impl<T, L> FromIterator<T> for LuaTable<L> {
    fn from_iter<I: IntoIterator<Item=T>>(iter: I) -> Self {
         // where do we take Lua from?
    }
}