Add local repo option

The problem this addresses is to allow people without jenkins repo access to build and modify the plugin

in the gradle.properties file change the line to
```
version=0.6.1-LOCAL
```
for example
```
./gradlew uploadArchives
```
will add the jar to build/repo

In a corresponding build.gradle in your actual jenkins plugin you can add
```    
buildscript {
    repositories {
        maven {
            name "jenkins-local"
            delegate.url( "file://path/to/gradle-jpi-plugin/build/repo" )
        }
        maven {
            name "jenkins"
            delegate.url("http://repo.jenkins-ci.org/releases/")
        }
        mavenCentral()
    }
    dependencies {
        classpath 'org.jenkins-ci.tools:gradle-jpi-plugin:0.6.1-LOCAL'
    }
}
```