Add init, remove, clean and server tasks

Added the following tasks:

- jbakeInit: uses the fidbake template to initialise the project
- jbakeRemove: removes all jbake files (deletes src/jbake)
- jbakeClean: deletes the generated content of jbake
- jbakeServer: runs the server

I added the missing runtime dependencies that you have to hunt around for that everyone will probably have to solve at some stage (jetty, freemarker, pegdown, asciidoctor, thymelfeaf).

The build gradle file I used to test the updated plugin is below.

build.gradle:
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buildscript {
    repositories {
        mavenLocal() // temporary: for updated jbake-gradle-plugin
        jcenter()
        // fidbake template
        maven {
            url "https://oss.sonatype.org/content/groups/public"
        }
    }
    dependencies {
        classpath "me.champeau.gradle:jbake-gradle-plugin:0.2.1-SNAPSHOT"
    }
}

apply plugin: 'me.champeau.jbake'

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