Improve the abstraction that's used for link extraction

Previously, the link extraction abstraction was inadequate as it made
assumptions about the format of the links that did not apply to all
media types. For example, it was suited to the links returned in a
application/hal+json response, but was not suited to the Atom-style
links often found in application/json responses.

This commit improves the abstraction so that the extracted links are
decoupled from the format of the response. In addition to the existing
support for extracting HAL links a new extractor for Atom-style links
has been introduced. Both extractors are now available via static
methods on the new LinkExtractors class. The sample applications have
been updated accordingly.

Closes #6

spring-restdocs-core/src/test/resources/link-payloads/atom/multiple-links-different-rels.json

@@ -0,0 +1,9 @@
+{
+	"links": [ {
+		"rel": "alpha",
+		"href": "http://alpha.example.com"
+	}, {
+		"rel": "bravo",
+		"href": "http://bravo.example.com"
+	} ]
+}

spring-restdocs-core/src/test/resources/link-payloads/atom/multiple-links-same-rels.json

@@ -0,0 +1,9 @@
+{
+	"links": [ {
+		"rel": "alpha",
+		"href": "http://alpha.example.com/one"
+	}, {
+		"rel": "alpha",
+		"href": "http://alpha.example.com/two"
+	} ]
+}

spring-restdocs-core/src/test/resources/link-payloads/atom/no-links.json

@@ -0,0 +1 @@
+{ }

spring-restdocs-core/src/test/resources/link-payloads/atom/single-link.json

@@ -0,0 +1,6 @@
+{
+	"links": [ {
+		"rel": "alpha",
+		"href": "http://alpha.example.com"
+	} ]
+}

spring-restdocs-core/src/test/resources/link-payloads/atom/wrong-format.json

@@ -0,0 +1,5 @@
+{
+	"_links": {
+		"alpha": ["http://alpha.example.com/one", "http://alpha.example.com/two"]
+	}
+}

spring-restdocs-core/src/test/resources/link-payloads/hal/multiple-links-different-rels.json

@@ -0,0 +1,6 @@
+{
+	"_links": {
+		"alpha": "http://alpha.example.com",
+		"bravo": "http://bravo.example.com"
+	}
+}

spring-restdocs-core/src/test/resources/link-payloads/hal/multiple-links-same-rels.json

@@ -0,0 +1,5 @@
+{
+	"_links": {
+		"alpha": ["http://alpha.example.com/one", "http://alpha.example.com/two"]
+	}
+}

spring-restdocs-core/src/test/resources/link-payloads/hal/no-links.json

@@ -0,0 +1 @@
+{ }

spring-restdocs-core/src/test/resources/link-payloads/hal/single-link.json

@@ -0,0 +1,5 @@
+{
+	"_links": {
+		"alpha": "http://alpha.example.com"
+	}
+}

spring-restdocs-core/src/test/resources/link-payloads/hal/wrong-format.json

@@ -0,0 +1,9 @@
+{
+	"_links": [ {
+		"rel": "alpha",
+		"href": "http://alpha.example.com/one"
+	}, {
+		"rel": "alpha",
+		"href": "http://alpha.example.com/two"
+	} ]
+}