Add a bracket validator

  * checks that all opened brackets are correctly closed
  * returns false if they are not, true if they are
  * O(n) time if we need to go through all of the brackets,
    which we would if the brackets are correct
  * O(n) space in case all of the brackets are openers and
    are all added to the stack
Merge pull request #1 from mknudsen01/bracket-validator

Add a bracket validator