Use field instead of an array in LocationMarker

A little tweak to avoid bashing GC.

Before:

Source|Size (KiB)|Time (ms)|Time (ms)|Time (ms)
:--:|--:|--:|--:|--:
Underscore 1.5.2|42.5| 4.96 ms ± 1.16%|  4.99 ms ± 1.66%|  4.94 ms ± 1.21%
Backbone 1.1.0|58.7| 5.22 ms ± 1.03%|  5.30 ms ± 0.77%|  5.16 ms ± 0.82%
MooTools 1.4.5|156.7|43.60 ms ± 3.90%| 43.88 ms ± 3.68%| 43.27 ms ± 3.79%
jQuery 1.9.1|262.1|58.02 ms ± 1.63%| 59.68 ms ± 2.11%| 58.27 ms ± 2.06%
YUI 3.12.0|330.4|45.46 ms ± 1.42%| 45.71 ms ± 1.62%| 45.02 ms ± 1.58%
jQuery.Mobile 1.4.2|442.2|98.91 ms ± 2.35%|101.79 ms ± 3.45%|100.41 ms ± 2.88%
Angular 1.2.5|701.7|77.79 ms ± 2.50%| 77.52 ms ± 2.38%| 78.46 ms ± 2.12%
Total|1994.3|333.95|338.87|335.52

After:

Source|Size (KiB)|Time (ms)|Time (ms)|Time (ms)
 :--:|--:|--:|--:|--:
Underscore 1.5.2|42.5| 4.63 ms ± 1.42%| 4.66 ms ± 0.88%| 4.67 ms ± 1.42%
Backbone 1.1.0|58.7| 5.08 ms ± 0.97%| 5.07 ms ± 0.52%| 4.98 ms ± 0.55%
MooTools 1.4.5|156.7|37.95 ms ± 6.46%|36.97 ms ± 6.26%|35.09 ms ± 6.43%
jQuery 1.9.1|262.1|58.76 ms ± 3.04%|55.08 ms ± 2.33%|55.85 ms ± 2.77%
YUI 3.12.0|330.4|34.60 ms ± 5.85%|32.89 ms ± 5.04%|34.15 ms ± 6.43%
jQuery.Mobile 1.4.2|442.2|96.50 ms ± 3.58%|97.23 ms ± 3.01%|92.88 ms ± 3.01%
Angular 1.2.5|701.7|72.55 ms ± 3.21%|77.85 ms ± 1.96%|75.76 ms ± 3.39%
Total|1994.3|310.07|309.74|303.38

Change:
 +9.22%