Make TraceKeys configurable

User can now configure additional HTTP headers to tag via
spring.sleuth.keys.http.headers.

Fixes gh-118

spring-cloud-sleuth-core/pom.xml

@@ -33,6 +33,11 @@
 			<artifactId>spring-boot-starter-web</artifactId>
 			<optional>true</optional>
 		</dependency>
+		<dependency>
+			<groupId>org.springframework.boot</groupId>
+			<artifactId>spring-boot-configuration-processor</artifactId>
+			<optional>true</optional>
+		</dependency>
 		<dependency>
 			<groupId>org.springframework.boot</groupId>
 			<artifactId>spring-boot-starter-actuator</artifactId>