New: enableAutoWidth/enableAutoHeight calculated off window proportions

Some variables have been changed to reflect/facilitate this.
Fixed: viewerWidthPadding is now symmetrical

Also took out a few lines accidentally introduced in the last commit.

Note to self - lines 2129 and 1376 are where to look to solve #134. May just call for complete removal.

Partially resolves #133.
Merge branch 'develop' of https://github.com/DDMAL/diva.js into develop