GROOVY-3457: Adding new StreamingTemplateEngine to address >64k templating issues

This code is ported from existing github repo at: 

https://github.com/mbjarland/groovy-streaming-template-engine

and has been mentioned in the [GROOVY-3457](https://jira.codehaus.org/browse/GROOVY-3457) ticket and recommended for a pull request by Cedric Champeau. The history and details of the code can be found in the readme of the above repository. 

As the code in the original repository was using the spock framework for unit testing and as I believe it would be a shame to devolve the unit tests back into junit, I added the spock framework as a dependency to the groovy-template subproject. 

I further added a checkstyle configuration which makes it possible to suppress checkstyle rules for specific lines of code using code comments. I realize this might be preposterous and against contribution rules and if so let me know. 

I did this as I believe there are cases where breaking a rule is the right thing to do. 

I received a broken build when trying to build the entire groovy-core project, but the groovy-templates subproject builds and tests fine (including the new unit tests added by this pull request).