2-3000% performance enhancement in IE7

Changed lib/dust.js to use an array for storing the chunk. Makes dust.js usable in IE7.

Our tests: 500 rows in a dust generated HTML table
- IE7 original dust: Up to 45-50 seconds (45000 - 50000 ms)
- Google Chrome and others: 25-30 ms
- IE7 with "chunk in array" implementation: 350-400 ms 
(original implementation ~12000% slower)

Other tests with less data show differences around 2-3000% .

No measurable changes in performance in other browsers like IE8-10, Chrome, Firefox etc

best regards
Johan Linderoth (jlkonsultab@github)
Nicklas Sigurdh (nicklassigurdh@github) 
Kent Pettersson (kent78@github)