Issue #128 Changed minimum node engine to >= 0.10

[ctarrington]

Resolve Issue 128

ursa@0.8.5 works for node 0.8 but not higher ursa@0.9.1 works for node 0.10 and up

This change captures these constraints in the engine section of package.json

[quartzjer]

Thanks! 0.9.2 published now as well :)