Add simple dependency injection container

Uncle Bob said "Depend on abstractions, not concretions". This is the D
in the SOLID principles of OO design[1]

A dependency injection container makes life a little easier when
following the dependency injection pattern. It holds dependency
configuration ("wiring"), and implements conventions for
fulfilling object dependencies, as follows:

1. constructor injection

   the container will use the names of constructor arguments and find
   dependencies with the corresponding name

2. struct member injection

   since we commonly use Structs as value objects, the container also
   supports struct members as dependency definitions

Different containers can be used in different contexts (for example in
production vs. test).

[1] http://en.wikipedia.org/wiki/SOLID_(object-oriented_design)
Make SpecialistDocumentRegistry an object instance
Specify behaviour when slug already taken
Merge pull request #7 from alphagov/dependency-injection

Dependency injection