Create ScriptHeaderService

Create a new ScriptHeaderService, a handler of a new ScriptHeader plugin
type.

The purpose of this new framework is to allow flexible addition of
header text to scripts, specified by language.
Use StringBuilder instead of concatenation

Pointed out by ctrueden: string concatenation has been showed to have
significant overhead in time and space, thus we should be using
StringBuilder.

This also removes the potential for a null header value.
Merge pull request #116 from scijava/script-header-service

ScriptHeader auto-generation