add usage error for invalid keys

[robertkowalski]

• keys must be a Buffer
• keys must be 32 bytes

this makes it easier for people to see what is wrogn when they receive

/Users/robert/bitfinex/network/node_modules/xor-distance/index.js:4
  if (a.length !== b.length) throw new Error('Inputs should have the same length')
                                   ^

Error: Inputs should have the same length
    at dist (/Users/robert/bitfinex/network/node_modules/xor-distance/index.js:4:36)

or:

/Users/robert/bitfinex/hcbench/node_modules/k-bucket/index.js:60
    throw new TypeError(name + ' is not a Uint8Array')
    ^

TypeError: id is not a Uint8Array
    at ensureInt8 (/Users/robert/bitfinex/hcbench/node_modules/k-bucket/index.js:60:11)

relaeted: #11

[davidmarkclements]

addressed in v2 refactor - thank you!