Added createComment for post, getComments only for specific post, default ids

[nordsimon]

Hi

Wanted to play around a bit with aws lamdba, dynamoDB etc and used this as an example togheter with the serverless CLI, great job with the tool!

Implemented create and get comment just for testing, it all works fine so try it out and merge if you want to

Also added default id generation for posts and comments (got tired of making up new ids)

Simon