removed missile_destruction_routes method
added position and children_count
added preorder method
method names anticipated by '#'
changed leftmost_children to leftmost_branch and more

These changes include changing the name of the #leftmost_children
method to #lefmost_branch, #rightmost_children to #rightmost_branch and
adding the #preorder method. Also, a little refactor.

Respective spec has changed to reflect the changes above.
added prune_strategy.rb and its respective spec
refactor branches methods
changed access to position and children_count
refactor add_child method
refactor to fulfill rubocop's advices
corrected typo
changed high to altitude
optimal missile route is now first route with greater size
changed test case to expect optimal destruction route as the first greater-size route
added empty anti_missile.rb
functional app with fixed input directory
receives input dir as first argument