Spring Boot does not notice spring.factories declarations in JARs added via -cp

Currently, if a `spring.factories` file is defined in either an application or a JAR included within a Spring Boot packaged application (e.g. via the Maven plugin), it's contents are read and executed as expected.  However, if a JAR is added to the classpath via the `cp` argument (e.g. `java -cp additional.jar -jar application.jar`), its `spring.factories` file is not detected and executed as expected.

A simple application demonstrating this behavior can be found at <https://github.com/nebhale/spring-boot-529>.  Given the a clone of the repository, this behavior can be demonstrated by running `mvn clean package && ./reproduce.sh`.

The output of this command currently looks like:

```bash
Executing: java -cp external-dependency/target/external-dependency-1-SNAPSHOT.jar -jar application/target/application-1-SNAPSHOT.jar

  .   ____          _            __ _ _
 /\\ / ___'_ __ _ _(_)_ __  __ _ \ \ \ \
( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
 \\/  ___)| |_)| | | | | || (_| |  ) ) ) )
  '  |____| .__|_| |_|_| |_\__, | / / / /
 =========|_|==============|___/=/_/_/_/
 :: Spring Boot ::            (v1.0.0.RC4)

2014-03-19 10:31:44.714  WARN 6486 --- [           main] c.n.LocalApplicationContextInitializer   : Executing LocalApplicationContextInitializer
2014-03-19 10:31:44.719  INFO 6486 --- [           main] com.nebhale.Application                  : Starting Application on bhale-desktop.local with PID 6486 (/Users/bhale/dev/sources/nebhale/spring-factories-loader-bug/application/target/application-1-SNAPSHOT.jar started by bhale)
```

Ideally, the output would look like:

```bash
Executing: java -cp external-dependency/target/external-dependency-1-SNAPSHOT.jar -jar application/target/application-1-SNAPSHOT.jar

  .   ____          _            __ _ _
 /\\ / ___'_ __ _ _(_)_ __  __ _ \ \ \ \
( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
 \\/  ___)| |_)| | | | | || (_| |  ) ) ) )
  '  |____| .__|_| |_|_| |_\__, | / / / /
 =========|_|==============|___/=/_/_/_/
 :: Spring Boot ::            (v1.0.0.RC4)

2014-03-19 10:30:16.548  WARN 6436 --- [           main] c.n.LocalApplicationContextInitializer   : Executing LocalApplicationContextInitializer
2014-03-19 10:30:16.552  WARN 6436 --- [           main] c.n.RemoteApplicationContextInitializer  : Executing RemoteApplicationContextInitializer
2014-03-19 10:30:16.553  INFO 6436 --- [           main] com.nebhale.Application                  : Starting Application on bhale-desktop.local with PID 6436 (/Users/bhale/dev/sources/nebhale/spring-factories-loader-bug/application/target/application-1-SNAPSHOT.jar started by bhale)
```

The initial discussions of this problem can be found in #322.