Chart export to bitmap (PNG, PDF, TIFF, JPG)

I implemented chart export functionality. It consists of client-side support (converting chart to SVG and posting it to the server) and a servlet in saiku that receives SVG (XML) and returns binary representation. Conversion is backed by [batik](http://xmlgraphics.apache.org/batik/) library, thus it supports PNG, PDF (exported in vector format, high quality zoom available), TIFF or JPG. Also we can download SVG file *as-is*.

Unfortunately server-side component was required (you may wish to port servlet to REST service). Hidden form is used to make sure all files pop-up with browser's *Save...* dialog rather than replacing current page with image.

#### Known issues:

* doesn't work on Google Chrome. Most likely some JavaScript issue.

This pull request contains all commits (first 8) from [*Multiple chart fixes*](https://github.com/OSBI/saiku-ui/pull/166). Last two implement the actual export on the client side.