node kind RPC

[rphmeier]

cc @jacogr for JS side of things.

Adds parity_nodeKind which returns an object

{
    "availability": "availability_str",
    "capability": "capability_str"
}

availability_str one of public, personal
capability_str one of full, light

We may add more variants later, but this is enough to cover current use cases of web and personal nodes.

[jacogr]

Just to get my ducks in a row -

1. public = web server (@maciejhirsz 's work), personal = nodes we all run daily
2. full = normal node, light = light client (@rphmeier 's work) (these 2 are pretty self-explanatory)

[rphmeier]

Yep, exactly

[gavofyork]

. might have broken protocol there - feel free to revert if the JS isn't up to scratch (LGTM though).