Issue Description:

It'd be nice to be able to define targets that are built only in test/dev environments, but not as part of a package.

Given a package that specifies test targets (note: actual API is TBD, but here's an example)

let package = Package(
  name: "Example",
  // etc
  targets: [ Target(name: "ExamplePkgTarget") ],
  testTargets: [
    Target(
      name: "ExampleTestTarget",
      dependencies: [
        .Dependency("some-test-dependency"),
        .Target(name: "ExamplePkgTarget")
      ]
    )
  ]
)

When I specify the package as a dependency, then ExamplePkgTarget should be built, and ExampleTestTarget should not be built

When the Example Package is the root package, then both ExamplePkgTarget and ExampleTestTarget should be built.

When a user specifies no test targets, assume (as we do now) that all targets will be built upon distribution.

Test targets should be allowed to link against testDependencies, while "normal" targets should not.