Image resize filters

This is a draft for filtered scaling in fabricjs.

I used the already existing filter interface.
The resize filter can be used as a normal filter in queque with other or just during rendering after a scaling event.

There are several type of filtering incluede, some of them are slow, speed differences between firefox and chrome is high on lanczos filter.

Some changes in imageclass to make possible sincronous rendering after filtering.

![image](https://cloud.githubusercontent.com/assets/1194048/5040141/8a130cce-6ba9-11e4-92a9-53e81b9c0c99.png)


Still need fixing to documentation and code style probably.

related issue: #1741
`sliceByTwo` would be better; the convention is usually to only uppercase classes/constructor