fixed #4 - step.depends

* default for step is to throw errors. If you want them passed to the
next step you need *@throw_errors = false*
* Removed implicit synchronous code from step. Just call @() at the end
of a synchronous step
* Added step.depends as a shortcut to multiple dependencies that may
have asynchronous initialisers. These dependencies must return (error,
next) -> function
* added step.libraries to load a list of js library modules. Callback
when all libraries loaded.
* fixed less so that @import will import from the same dir as the less
file.