643 - improve text measurer

First step: make usage to textMeasure only take in text selections
643 - TextMeasurer takes in a temporary text node to measure against
Aligning usage to be in line with new getTextMeasure 
  Now measurers need only be created once and can be reused
Updating jsdoc on getTextMeasure
getTextMeasure -> getTextMeasurer

Close #643.
no longer need to `return this;` as of #810