Adding support for Dynamic Entities

[salvatorecordiano]

Reference url: https://developer.amazon.com/en-US/docs/alexa/custom-skills/use-dynamic-entities-for-customized-interactions.html

[coveralls]

Coverage increased (+0.1%) to 95.995% when pulling cf60721 on salvatorecordiano:master into fd08ce5 on maxbeckers:master.

[salvatorecordiano]

Ready for review! @maxbeckers

[maxbeckers]

Thanks for your work, seems good. Just one little thing i commented.

[salvatorecordiano]

@maxbeckers can you tag this version, please?