Add .let to define memoized "virtual" methods on presenter resource

[kainosnoema]

For some reason the .let and .lets methods were added in f312bc8, but they weren't implemented. This completes a basic implementation which defines "virtual" memoized methods on the resource.

• Efficient reuse of expensive values in multiple properties
• Cleans up model decorators even further

Example:

class UserPresenter < Jsonite
  let(:full_name) { "#{first_name} #{last_name}" }
  property :full_name
end
# {
#   "full_name": "Stephen Celis"
# }

Downsides:

• Third place where presentation methods are defined (context, resource, lets)