Add api.util.encodeMethodCall to parity.js

[jacogr]

Closes https://github.com/ethcore/parity/issues/4325

[jacogr]

Not 100% convinced the options are needed for the extra options.data in all the use-cases.

[gavofyork]

from #4325:

would be good to have two (basically) symmetric calls:

• parity.api.util.abiEncode(name, types, params) -> data (name may be null, in which case it skips the function sig, making it useful for encoding events/constructor/fallback)
• parity.api.util.abiDecode(types, data) -> params
  Here's my polyfill for the current PR:

parity.api.util.abiEncode = function(name, types, data) {
   let r = parity.api.util.encodeMethodCall(name || '', types, data);
   return name === null ? `0x${r.substr(10)}` : r;
};
parity.api.util.abiDecode = function (types, data) {
   return parity.api.util.decodeMethodInput({inputs: types.map(t => {return {type: t};})}, data);
};

[jacogr]

Added abiEncode & abiDecode (99.9% copy from provided, just tests added)