Issue #420 Implemented MkGitignores.iterate()

Since the [Gitignores Template API](http://developer.github.com/v3/gitignore/) pretty much consists of returning static content with no support for creating new templates or editing existing ones, I thought it would be reasonable to just use hard-coded mappings instead of creating new `MkStorage` entries. For now it contains just one template for Java, but we can add new ones later.

Let me know if you think the approach is incorrect.