Switch to sekizai rather than existing template tags

@martey This is probably one up for some discussion!

I decided to replace all of the existing template tag implementation with a sekizai based solution, at the cost of another dependency I think personally that the advantages outweigh the extra dependency.

For:
django-compressor support for the js output, the ability to use RequestContext without crashes, less code to maintain, localised js sdk, clean/logical templates

Against:
Extra dependency, 2 additional context processors required, using RequestContext is a requirement to have all applicable variables available.

Please have a look over 86046bf - if you are interested in merging this into the mainline, then i'll rebase it and remove all of the other nonsense from my other PR.