Add a header option to the ant task to prepend source code files

[alfonsodelavega]

Currently, sjpp removes any header from source code files, and places the generic message // THIS FILE HAS BEEN GENERATED BY A PREPROCESSOR.

Having the option to e.g. read an input file to use as header will allow customising it. The distribution of PlantUML codebase under different licenses is an existing use case for this feature, such as:

<target name="dist">
	<sjpp src="src-folder" dest="src-generated" define="__MIT__"  header="mit_header_content.txt" />
</target>

I'll submit a PR with this functionality shortly.