Improve performance by skipping elements
Improve performance by skipping elements

When scanning for landmarks, only consider nodes that are *element*
children of other nodes (previously all types of nodes were visited, but
non-element nodes were discounted). This also, and I expect
significantly, replaces the forEach function calls with a for loop,
which will be more efficient.

==> r0-control.json <==
{
  "abootstrap": {
    "mean": 93,
    "standardDeviation": 19
  },
  "amazon": {
    "mean": 9.2,
    "standardDeviation": 3.3
  },
  "ars": {
    "mean": 5.7,
    "standardDeviation": 2
  },
  "bbcnews": {
    "mean": 7.9,
    "standardDeviation": 2.8
  },
  "googledoc": {
    "mean": 1.3,
    "standardDeviation": 1.8
  }
}
==> r1-test.json <==
{
  "abootstrap": {
    "mean": 70,
    "standardDeviation": 17
  },
  "amazon": {
    "mean": 7.8,
    "standardDeviation": 3.4
  },
  "ars": {
    "mean": 4.6,
    "standardDeviation": 2.1
  },
  "bbcnews": {
    "mean": 7,
    "standardDeviation": 3.2
  },
  "googledoc": {
    "mean": 1.4,
    "standardDeviation": 1.9
  }
}

Fixes #126.

matatk/landmarks,matatk/landmarks,matatk/landmarks