Support CoffeeScript source map generation

JavaScript SourceMaps provide mapping between compiled JavaScript
sources and original code (which may be CoffeeScript or JS before
minification). It is implemented in Chrome and currently making
its way into FireFox.

Source map work as follow:
- compiler (CoffeeScript) generate a map file (e.g. source.map)
  containing mapping information between the original and the
  generated source code,
- compiled js code should end by a comment specifying the mapping
  filename (e.g. //@ sourceMappingURL=source.map)
- original source file should be accessible by the browser.

CoffeeScript >= 1.6.1 support source maps generation, however,
CoffeeScript == 1.6.2 fix a line numbering issue in the map file.

Details regarding the implementation:
- in Build.scala, coffeescriptOptions should contains a "map" option,
- CoffeescriptCompiler.compile() call JavaScript CoffeeScript.compile()
  with the appropriate options, and convert the returned results,
- CoffeescriptCompiler.compiler() and
  PlayAssetsCompiler.AssetsCompiler()'s `compile` argument return a new
  Option containing the generated source map within a string,
- map file and original source files are written in the resource_managed
  directory and names are returned so to be found by Assets,
- sourceMappingURL comment is added by CoffeescriptCompiler helper
  (it is not added by CoffeeScript library until the output file is
  written),
- JavaScript exceptions handling uses the exception.location property to
  retrieve the error's lines and columns numbers
  (new since CoffeeScript 1.6.2),
- tests are implemented in CoffeeCompilerSpec.scala.

Documentation has been updated in AssetsCoffeeScript.md