Common graph constructor support

Added a new namespace: Evolve2.GraphTypeHelpers which contains helpers
for creating common graphs types. Current helpers are: chain, clique and
burst. Each helper has a default implementation (ie Chain) and a
generically typed implementation (Chain<T>).

Added new namespace Evolve2.Util.Factories which contains interfaces and
default implementations of vertex and edge factories.

Renamed GuidIdentityProvider to DefaultIdentityProvider to fit with the
new naming convention.