implement League/URI for id:elem generation

[jrushlow]

Currently, Entry & Feed constructors set the provided $id param directly to $id properties. On get(), $this-id is checked that it's not an empty string and returned.

In the FeedFactory, the $id param is checked in validateRequiredFeedElements() to ensure its not an empty string.

PR #21 implemented URI/IRI validation for id:elements provides RFC 3986 & 3987 compliance/validation for the $id param (Atom id:element).

It would be better to utilize League/Uri::createFromString() when supplying the $id constructor param in factories to help ensure a model object always contains a valid & compliant $id property.

[jrushlow]

Note to self:
Implement Uri::createFromString() in the Model directly?
Would this be possible without causing the constructor to do work rather than provide params?
Would this restrict the ability for outside libraries using Atom\Models* to utilize their own RFC 3987 RFC 3986 validation? Or enforce the concept upon them that Atom\Models* are always valid objects upon creation?