Fixed multiple listeners with `once()`

When `#once()` was used, in `#_emit()` `#splice()` was called on the array
of callbacks, but `i` and `len` where not updated, producing errors.
Added tests for multiple listeners with `#once()`
Changed test to karma-mocha & coverage to istanbul

Now using karma runner to tests as ak-* micro-libs are used both in node
and the browsers.
Thus using istanbul for code coverage, as karma-mocha is not handling
html-cov reporter. Moreover, istanbul seems much better though the UI is
far from being as sexy as mocha's.
Release 0.0.3