slip in PATH for 'npm test' as not everyone has karma

So people can follow the 'unit tests' instructions, we need
to make sure PATH includes the karma bin directory otherwise
we see the following:
```
aclouter@stevemcqueen:/usr/src/aten-ikvm/noVNC$ npm test

> noVNC@0.5.0 test /usr/src/aten-ikvm/noVNC
> karma start karma.conf.js

sh: 1: karma: not found
npm ERR! Test failed.  See above for more details.
npm ERR! not ok code 0
```