Tweak README wording
Remove Elements namespace

Iora::Elements::Element was renamed to simply Iora::Element, as were all
classes in the Iora::Elements namespace (this namespace is now gone,
even though the folder is still there). This is a purely aesthetic
change.
Update Gemfile to use YARD branch "frameless"
Move relation DSL methods to Iora::Relations
Remove core extensions

People aren’t going to want to use Iora if it pollutes external objects
with mysterious methods (like Array#search) and constants (like
Boolean, now Iora::Boolean).
Add convenience method Coercer.coerce
Clean up doc comments for Element
Make Coercer.coerce return if passed a nil value

This eliminates an otherwise necessary nil check in the code that’s
doing the coercing.
Move relation methods into the Element class

Because it relies on the element instance variable, it doesn’t make
sense to have the methods separate. This change also makes it much
easier to extract shared code between relations into instance methods
on Element.
Remove old relations modules
Fix YARD warning
Clean up Result element methods
Don't specify a tag for nested nodes

This change was made because the keyword "tag" suggests a single tag
name, not a css selector. While this way might have the tiniest
performance hit, it also removes a good amount of code.
Use has_one and has_many DSL methods
Use overhanging style in doc comments
Use Boolean in doc comments
Refactor Element.coerce_node
Clarify doc comment in Coercer#to_boolean
Clarify documentation on errors
Make element tests consistent