do not force default 300x150 if no width/height found

Hello,

This PR is one step forward to responsive design. It's pretty straight-forward  - set width, height on container only if it's set. And default 300x150 dimensions are removed. It helps to control container dimensions totally from css. I know it's possible to use `!important` keyword in css but this approach is cleaner IMHO.

It can be considered breaking change but not sure if anyone is using default 300x150.

Looking forward for comments.

Thanks,
Alex