Added basic support for Gradle incremental tasks (issue cursive-ide#10).

When Gradle detects there were no changes to the input files, it won't call Clojure compile task at all.

src/main/kotlin/cursive/ClojurePlugin.kt

@@ -34,6 +34,7 @@ import org.gradle.api.plugins.JavaPluginConvention
 import org.gradle.api.tasks.SourceSet
 import org.gradle.api.tasks.TaskAction
 import org.gradle.api.tasks.compile.AbstractCompile
+import org.gradle.api.tasks.incremental.IncrementalTaskInputs
 import org.gradle.process.JavaForkOptions
 import org.gradle.process.internal.DefaultJavaForkOptions
 import org.gradle.process.internal.ExecException
@@ -65,7 +66,6 @@ class ClojurePlugin : Plugin<Project> {
       conventionMapping.map("classpath", {
         mainSourceSet.compileClasspath
             .plus(SimpleFileCollection(mainSourceSet.allSource.srcDirs))
-            .plus(SimpleFileCollection(mainSourceSet.output.classesDir))
       })
       conventionMapping.map("namespaces", {
         mainCompileTask.findNamespaces()
@@ -205,6 +205,10 @@ open class ClojureCompiler @Inject constructor(val fileResolver: FileResolver) :
   }
 
   @TaskAction
+  fun compile(inputs: IncrementalTaskInputs) {
+    compile()
+  }
+
   override fun compile() {
     logger.info("Starting ClojureCompiler task")
 
@@ -298,7 +302,7 @@ open class ClojureCompiler @Inject constructor(val fileResolver: FileResolver) :
     val exec = JavaExecHandleBuilder(fileResolver)
     copyTo(exec)
     exec.main = "clojure.main"
-    exec.classpath = classpath
+    exec.classpath = classpath.plus(SimpleFileCollection(destinationDir))
     exec.setArgs(listOf("-i", file.canonicalPath))
     exec.defaultCharacterEncoding = "UTF8"