Make the goal buffer length 30s

Expose the goal buffer length as a plugin "global" and bump its value up from 5 to 30s. Modifying the default value is not recommended. Update tests to succeed even if the goal buffer length is modified. For #63.

@@ -11,9 +11,6 @@
 
 var
 
-  // the desired length of video to maintain in the buffer, in seconds
-  goalBufferLength = 5,
-
   // a fudge factor to apply to advertised playlist bitrates to account for
   // temporary flucations in client bandwidth
   bandwidthVariance = 1.1,
@@ -370,7 +367,8 @@ var
 
       // if there is plenty of content in the buffer and we're not
       // seeking, relax for awhile
-      if (typeof offset !== 'number' && bufferedTime >= goalBufferLength) {
+      if (typeof offset !== 'number' &&
+          bufferedTime >= videojs.Hls.GOAL_BUFFER_LENGTH) {
         return;
       }
 
@@ -577,6 +575,9 @@ videojs.Hls = videojs.Flash.extend({
   }
 });
 
+// the desired length of video to maintain in the buffer, in seconds
+videojs.Hls.GOAL_BUFFER_LENGTH = 30;
+
 videojs.Hls.prototype.src = function(src) {
   var
     player = this.player(),

@@ -631,15 +631,16 @@ test('selects the correct rendition by player dimensions', function() {
 
 
 test('does not download the next segment if the buffer is full', function() {
+  var currentTime = 15;
   player.src({
     src: 'manifest/media.m3u8',
     type: 'application/vnd.apple.mpegurl'
   });
   player.currentTime = function() {
-    return 15;
+    return currentTime;
   };
   player.buffered = function() {
-    return videojs.createTimeRange(0, 20);
+    return videojs.createTimeRange(0, currentTime + videojs.Hls.GOAL_BUFFER_LENGTH);
   };
   player.hls.mediaSource.trigger({
     type: 'sourceopen'