Updating build script

A slightly better build

+ the source map now links back to the original coffee-script, so you can debug against that rather than the generated javascript.
+ the source map is in a separate file, so it will only need loading when debugging.

- the source map still has the full path to the location of the source files used to build it. So starts like this (on a windows build):
```
{
  "version": 3,
  "file": "generated.js",
  "sources": [
    "C:\\github\\wiki-client\\node_modules\\grunt-browserify\\node_modules\\browserify\\node_modules\\browser-pack\\_prelude.js",
    "C:\\github\\wiki-client\\client.coffee",
    "C:\\github\\wiki-client\\lib\\active.coffee",
...
```
I can't currently see a way of getting this to use relative paths with the currently available grunt tool-set. There are some hints that it may be supported in a future version of grunt-browserify. It also looks to be possible using the browserify api.

There are some tools that claim to be able to remove the absolute paths, but the ones looked at so far all fail as ```generated.js``` does not exist!