Adds excludeDependencies. Fixes #1748

Adds project-level dependency exclusions:

  excludeDependencies += "org.apache.logging.log4j"
  excludeDependencies += "com.example" %% "foo"

In the first example, all artifacts from the organization
`"org.apache.logging.log4j"` are excluded from the managed dependency.
In the second example, artifacts with the organization `"com.example"`
and the name `"foo"` cross versioned to the current `scalaVersion` are
excluded.
Merge pull request #1748 from sbt/wip/1748

excludeDependencies: excludeAll at the project level