Allow debugging unminified source code. Update README.md

There is no clearly defined and documented way to debug unminified dust-helpers.js code. Adding this ability with `grunt testClient` task. It is the same approach as in main dust repo (although testClient task uses unminified source file there).

Additionally coverage report generated by jasmine task was based on minified dust-helper.js file. It was hard to tell which lines were covered, since all of it was just one line. Moved out coverage task as a separate task and made it generate report based on unminified source code.

Added `grunt watch` task to be in line with main dust project and just because it's handy :)

Also update README.md to provide instructions on how all of these tasks should be used.

With these updates travis build will run unit tests against minified dust helpers code in Phantom, Node and Rhino. It will also run coverage report against unminified source code. If coverage thresholds are not met, build will fail.