Use field instead of an array in LocationMarker

A little tweak to avoid bashing GC.

Before:

|Source|Size (KiB)|Time (ms)|
|:-:|--:|--:|
|Underscore 1.5.2|42.5|   5.78 ms ± 1.83%|
|Backbone 1.1.0|58.7|   5.93 ms ± 0.85%|
|MooTools 1.4.5|156.7|  48.02 ms ± 4.08%|
|jQuery 1.9.1|262.1|  66.56 ms ± 2.48%|
|YUI 3.12.0|330.4|  52.90 ms ± 1.51%|
|jQuery.Mobile 1.4.2|442.2| 120.11 ms ± 3.40%|
|Angular 1.2.5|701.7|  90.88 ms ± 2.72%|
|Total|1994.3|390.17|

After:

|Source|Size (KiB)|Time (ms)|
|:-:|--:|--:|
|Underscore 1.5.2|42.5|  5.20 ms ± 1.35%|
|Backbone 1.1.0|58.7|  5.69 ms ± 0.88%|
|MooTools 1.4.5|156.7| 39.07 ms ± 6.19%|
|jQuery 1.9.1|262.1| 60.55 ms ± 1.88%|
|YUI 3.12.0|330.4| 35.63 ms ± 6.77%|
|jQuery.Mobile 1.4.2|442.2|106.35 ms ± 3.14%|
|Angular 1.2.5|701.7| 86.39 ms ± 3.45%|
|Total|1994.3|338.87|

Change:
15.14%