Added method Resolve and ResolveByName

[emanuel-v-r]

In order to be able to get previously provided instances on demand I have created the method "Resolve" and "ResolveByName". You can pass a pointer of a interface variable or struct variable.
This also satisfies the issue #19

The usage should be something like this:

var g inject.Graph
a := &TypeAnswerStruct{answer: 1, private: 2}
g.Provide(&inject.Object{Value: a})

var result Answerable

err := g.Resolve(&result)

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