Group: better error message for .expose()
Pump: added .errorBuffer method

Error buffer can be set or get by using .errorBuffer method. A buffer is
created automatically on .start() if not set. Any error not catched by the
.process() method will be written into the .errorBuffer() until it is filled
up.
Pump: code cleanup
Group: added errorbuffer handling

The group sets its own errorbuffer to all of its pumps before starting
them.
Updated js files

Forgot to run grunt watch in the bg, so js files were not updated.
Buffer: code cleanup
Group: emit error event when error buffer is full