fix some minor errors encountered with destroy and bmap text

Small inconsistency between Text and BitmapText ...

setting a text now use toString(), like PIXI.Text.

Move items into their namespace ...

This commit also:

 - fixes some dangling whitespace,
 - fixes some incorrect docs in extras/mesh,
 - moves Text to core
 - moves BitmapText to extras
 - moves math constants to const.js