Add a command to produce a self-contained executable JAR for a CLI app

A new command, jar, has been added to the CLI. The command can be
used to create a self-contained executable JAR file from a CLI app.

Basic usage is:

spring jar <jar-name> <source-files>

For example:

spring jar my-app.jar *.groovy

The resulting jar will contain the classes generated by compiling the
source files, all of the application's dependencies, and entries
on the application's classpath.

By default a CLI application has the current working directory on
its classpath. This can be overridden using the --classpath option.
Any file that is referenced directly by the classpath is always
included in the jar. Any file that is found a result of being
contained within a directory that is on the classpath is subject to
filtering to determine whether or not it should be included. The
default includes are public/**, static/**, resources/**,
META-INF/**, *. The default excludes are .*, repository/**, build/**,
target/**. To be included in the jar, a file must match one of the
includes and none of the excludes. The filters can be overridden using
the --include and --exclude options.

Closes #241

spring-boot-tools/spring-boot-loader-tools/src/main/java/org/springframework/boot/loader/tools/JarWriter.java

@@ -45,7 +45,7 @@ import java.util.zip.ZipEntry;
  * 
  * @author Phillip Webb
  */
-class JarWriter {
+public class JarWriter {
 
 	private static final String NESTED_LOADER_JAR = "META-INF/loader/spring-boot-loader.jar";
 
@@ -107,6 +107,18 @@ class JarWriter {
 		}
 	}
 
+	/**
+	 * Writes an entry. The {@code inputStream} is closed once the entry has been written
+	 * 
+	 * @param entryName The name of the entry
+	 * @param inputStream The stream from which the entry's data can be read
+	 * @throws IOException if the write fails
+	 */
+	public void writeEntry(String entryName, InputStream inputStream) throws IOException {
+		JarEntry entry = new JarEntry(entryName);
+		writeEntry(entry, new InputStreamEntryWriter(inputStream, true));
+	}
+
 	/**
 	 * Write a nested library.
 	 * @param destination the destination of the library