Add profile metadata

Adds :leaky and :pom-scope metadata for profiles.  The :dev, :test,
:base and :provided profiles are implemented in terms of these.

Profiles with :leaky metadata affect the pom and jar creation.

Profiles with a :pom-scope metadata of :test or :provided also affect
the dependencies of a pom.
Merge remote-tracking branch 'hugoduncan/feature/annotated-profiles'
Rename :core-default to :leiningen/default.
A few documentation changes.
Bump to 2.5.0-SNAPSHOT.