Change md5 implementation to checksum timestamps instead of files.

modules/kobalt-plugin-api/src/main/kotlin/com/beust/kobalt/maven/Md5.kt

@@ -18,7 +18,13 @@ public class Md5 {
 //            return DatatypeConverter.printHexBinary(md5.digest()).toLowerCase()
 //        }
 
-        fun toMd5Directories(directories: List<File>) : String? {
+        /**
+         * Calculate a checksum for all the files under the passed directories. The conversion from File to
+         * bytes can be customized by the @param{toBytes} parameter. The default implementation calculates
+         * a checksum of the last modified timestamp.
+         */
+        fun toMd5Directories(directories: List<File>,
+                toBytes: (File) -> ByteArray = { it.lastModified().toString().toByteArray() } ): String? {
             val ds = directories.filter { it.exists() }
             if (ds.size > 0) {
                 MessageDigest.getInstance("MD5").let { md5 ->
@@ -37,7 +43,7 @@ public class Md5 {
                                 it.isFile
                             }.forEach {
                                 fileCount++
-                                val bytes = it.readBytes()
+                                val bytes = toBytes(it)
                                 md5.update(bytes, 0, bytes.size)
                             }
                         }