Add ability to add custom header definitions

[arnarpall]

I added a way to define custom header definitions through the gradle dsl

Adding header definitions can be done throught he license extension
This more or less fits with the way it was done in the maven plugin
http://code.mycila.com/license-maven-plugin/#changing-header-style-definitions

license {
    headerDefinitions {
        custom_definition {
          firstLine = "//"
          endLine   = "//"
          firstLineDetectionPattern = "//"
          lastLineDetectionPattern  = "//"
          allowBlankLines = false
          skipLinePattern = "//"
          isMultiline = false
        }
    }
}

[hierynomus]

Wow, great thanks!
On first glance, only some styling issues. In some places you put the open-curly on a new line and other places you (correctly) didn't. If you could reformat that to match the style of the rest of the project, I think we're good to go!

[arnarpall]

No problem, i'll get that fixed

[hierynomus]

Thanks!