update README.md
yet another update
more informative message for presence invariant of Boolean attrs
fixed typo
fix exception message
add sorting while generate autoload files
move codings to handmade

Conflicts:
	lib/fhir/types/autoloads.rb
Merge branch 'master' of github.com:hospital-systems/fhir

Conflicts:
	lib/fhir/types/autoloads.rb
add spec for coding
whitespace fix
make DataType Virtus::ValueObject
broken spec for data types
use Fhir::URI in resource references
add Coding#to_concept
Merge branch 'master' of github.com:hospital-systems/fhir
add last_modified_date attribute to resource base type
fixed #serialize method
move codeable concept to handmade
Merge branch 'master' of github.com:hospital-systems/fhir
symbolizing keys in custom FHIR coercer