Add path defaults to all commands

[daTokenizer]

for ease of use, the default path would always be the root as defined by Path.rootPath()

[coveralls]

Coverage remained the same at 100.0% when pulling 7bf76ea on daTokenizer:master into 5e1ea9a on RedisLabs:master.

[itamarhaber]

Defaulting to the root path makes sense imo.

This PR, however, also bundles a new command, jsonmgetl, which accepts a list of key names instead of the existing variadic format of jsonmget. This I like less :) As a compromise, I can live with how redis-py's mget works, by accepting both a list and arguments. However, I'd like to see that come in as a separate change to the code, tests and documentation.

[itamarhaber]

Separate PR

[itamarhaber]

Separate PR - jsonmget's signature should change (breaking bw compat) to jsonmget(self, path=Path.rootPath(), keys=[], *args)