Text.js: Fixes, Docs

Summary:
-- Text.js no longer supports scaling.*
-- Text.js now also has all of its functions documented in JSDocs
format.

*Because Sprite.js uses its internal image property to create its
scaling, it can rely on the fact that the property is basically a
constant in all calculations. Its internal width and height can be
scaled according to a factor multiplied by the size of the image (which
never changes).

Text.js doesn't have that same relationship to its width and height. It
is drawn /in/ and not /from/ its dimensions. This means that, should
scaling be enabled, it would constantly be in flux as each new call to
cacheOffsets would increase or decrease its internal width and height
without the weight of something like an image.width to keep it as a
factor.

It could still be added back in the future though. For the most accurate
measurements, it should probably use the context.measureText for width
and context.font for height values. However, both are relative to the
font family used -- and the current object uses pixels instead of points
anyway for size consistency.