Replace commit signing with CLA app

Remove references to commit signing from the CONTRIBUTING.md guide and
add a link to StrongLoop's online CLA tracker:
https://strongloop-cla.herokuapp.com/agreements/strongloop/strongloop
This was a demo, but it could be merged.. Good catch on the UTF-8 change! I copied the file from the loopback repo where I had originally demoed, and that file is UTF-8 (that single character is, at least).