Add basic support for using bracket notation in JSON field paths

Previously, only dot notation JSON field paths were supported. Using
dot notation, each key is separated by a '.'. This makes it impossible
to reference a field where the key itself contains a dot.

This commit adds basic bracket notation support to JsonFieldPath.
With this commit, existing functionality remains as is, but users can
now use a basic form of JsonPath bracket notation. This enables the
use of Json keys with embedded dots. i.e. something like :

{
	"a.b" : "one"
}

.andDo(document("example",responseFields(fieldWithPath("['a.b']")…

Closes gh-132

spring-restdocs-core/src/test/java/org/springframework/restdocs/payload/JsonFieldPathTests.java

@@ -27,6 +27,7 @@ import org.junit.Test;
  * Tests for {@link JsonFieldPath}
  * 
  * @author Andy Wilkinson
+ * @author Jeremy Rickard
  */
 public class JsonFieldPathTests {
 
@@ -110,4 +111,19 @@ public class JsonFieldPathTests {
 				contains("[]", "a", "b", "c"));
 	}
 
+	@Test
+	public void compilationOfMultipleElementPathWithBrackets() {
+		assertThat(JsonFieldPath.compile("['a']['b']['c']").getSegments(), contains("a", "b", "c"));
+	}
+
+	@Test
+	public void compilationOfMultipleElementPathWithAndWithoutBrackets() {
+		assertThat(JsonFieldPath.compile("['a'][].b['c']").getSegments(), contains("a", "[]", "b", "c"));
+	}
+
+	@Test
+	public void compilationOfMultipleElementPathWithAndWithoutBracketsAndEmbeddedDots() {
+		assertThat(JsonFieldPath.compile("['a.key'][].b['c']").getSegments(), contains("a.key", "[]", "b", "c"));
+	}
+
 }