gradle-plugin: Specify compiler version in project properties

This patch allows gradle-plugin user to specify a compiler version
via project properties. The default version is 0.2. It also updates
samples to use this new version of gradle plugin.

samples/libcurl/build.gradle

@@ -7,7 +7,7 @@ buildscript {
     }
 
     dependencies {
-        classpath "org.jetbrains.kotlin:kotlin-native-gradle-plugin:0.1"
+        classpath "org.jetbrains.kotlin:kotlin-native-gradle-plugin:0.2"
     }
 }