Deprecate Relations.groupBy.

Also add equals/hashCode implementations for MRelations.

Also add some comments to explain that ++ and -- are naively implemented.

Also fix some tabs-vs-spaces indentation nits.
Work around various issues with Maven local repositories.  Fixes #321.

* when mvn does a local 'install', it doesn't update the pom.xml last modified time if the pom.xml content hasn't changed
* an ivy.xml includes publicationDate, so an ivy.xml will always be touched even if the other content hasn't changed
* when Ivy checks if a snapshot is uptodate
  + it sees a SNAPSHOT, so it knows the module metadata and artifacts might change
  + it then checks the lastModified time of the metadata
  + if unchanged, it uses the cached information
  + if useOrigin is effectively false (either it is explicitly false or a resource is remote/isLocal=false),
    this means that a new artifact won't be retrieved
* the Ivy IBiblioResolver
  + must be used for Maven repositories for proper behavior (no FileResolver, for example)
  + only returns URLResources, even for file: URLs
  + a FileResource is needed in combination with useOrigin to avoid copying artifacts from .m2/repository/

This commit fixes the above by setting a custom URLRepository on a constructed IBiblioResolver.
This URLRepository returns FileResources for file: URLs and standard URLResources for others.
The returned FileResource has isLocal=true and sbt sets useOrigin=true by default, so the artifacts
are used from the origin.

If it turns out a similar situation happens when mvn publishes to remote repositories, it is likely the fix for
that would be to figure out how to disable the lastModified check on the metadata and always download the metadata.
This would be slower, however.
fix dependency-management/cache-classifers test

used user cache, which made it flaky
Merge remote-tracking branch 'benjy/relations' into 0.13