Fix build.

.gitignore

@@ -9,6 +9,6 @@ local.properties
 classes
 libs
 .kobalt/
-build/
+./build/
 out
 .DS_Store

modules/kobalt-plugin-api/src/main/kotlin/com/beust/kobalt/internal/build/BuildFile.kt

@@ -0,0 +1,17 @@
+package com.beust.kobalt.internal.build
+
+import java.io.File
+import java.nio.file.Files
+import java.nio.file.Path
+import java.nio.file.attribute.BasicFileAttributes
+/**
+ * Sometimes, build files are moved to temporary files, so we give them a specific name for clarity.
+ */
+class BuildFile(val path: Path, val name: String) {
+    public fun exists() : Boolean = Files.exists(path)
+
+    public val lastModified : Long
+        get() = Files.readAttributes(path, BasicFileAttributes::class.java).lastModifiedTime().toMillis()
+
+    public val directory : File get() = path.toFile().directory
+}

modules/kobalt-plugin-api/src/main/kotlin/com/beust/kobalt/internal/build/VersionFile.kt

@@ -0,0 +1,20 @@
+package com.beust.kobalt.internal.build
+
+import com.beust.kobalt.api.Kobalt
+import com.beust.kobalt.misc.KFiles
+import java.io.File
+
+class VersionFile {
+    companion object {
+        private val VERSION_FILE = "version.txt"
+
+        fun generateVersionFile(directory: File) {
+            KFiles.saveFile(File(directory, VERSION_FILE), Kobalt.version)
+        }
+
+        fun isSameVersionFile(directory: File) =
+            with(File(directory, VERSION_FILE)) {
+                ! exists() || (exists() && readText() == Kobalt.version)
+            }
+    }
+}