extension support so that other plugins can contribute to the plugin

that for specific files a schema can be returned.

currently it is quite hard coded to use ".json" files and that those have a schema right besides it.

we don't want a schema file right besides it, We base everyting on a different extension ".spec" as an example
And all spec files have the same schema, Those spec files can be in many dirs. 

With this patch our plugin that uses the json validation plugin can just implement a eclipe extension point and contribute a class that test itself if it wants to give a schema based on the given file.

the last part in "checkAgainst" is for the latest json validation jar. You can skip this if needed.

(i did move the parse line numbers to that area also because that only has to be done if there are warnings/errors found)