External dependencies docs: Write "string notation" in lower case
External dependencies docs: Remove duplicate string notation example

The removed example is immediately followed by a more comprehensive
example that also includes the "classifier".
External dependencies docs: Extend the examples with the optional "ext"

Extend the examples with the "Artifact only notation" as described at

http://www.gradle.org/docs/current/userguide/dependency_management.html#ssub:artifact_dependencies
IvyArtifactRepository: Add a link to available Ivy pattern tokens