Doc: Bring groovy versions back into alignment

Minor change to userguide to bring groovy versions back into alignment.
Documentation fix: your are -> you are
Added support for bintray's jcenter repository
Merge branch 'master' of https://github.com/gradle/gradle
Integration test for jcenter resolution
Added a description for jcenter resolution and a minimal background about jcenter
distributionUrl defaults should be Groovy strings due to use of placeholder syntax
Some spec updates.
Reordered some c++ stories.
Fleshed out some sections of the release notes.
Some spec updates.
Added a little more detail to the release notes.
Merge pull request #168 from dstine/wrapper-docs

docs for default distributionUrl should use Groovy strings
Merge pull request #159 from sgoings/master

Trivial documentation update
Replace MappingCopySpecVisitor with CopySpecVisitorDriver.

MappingCopySpecVisitor was previously the adapter between the FileVisitor world and the CopySpecVisitor world which created some awkwardness. This allows an upcoming change to make CopySpecVisitor not implement FileVisitor which detangles some things.
Don't have CopySpecVisitor extend FileVisitor.
Merge pull request #164 from yoav/master

Bintray JCenter Support
Project dependencies map to a single publication of the depended-on project
- For single publication, use it
- For multiple publications, fail unless all publications have the same coordinates
- For no publications, use the project coordinates
- Added ProjectDependencyPublicationResolver to do the work of determining the
  coordinates to use for a project dependency
Merge pull request #145 from sbennettmcleish/master

Minor userguide update for Groovy version.
Improved the content of files generated by the build setup plugin

- Escape values differently based on whether they are injected into a comment or a string
- Some reworking of the comments in the template files.