Added the ability to override non-zero values

[apourchet]

Please note that this patch does not break any of the existing functionalities
or guarantees of the package; it is only a collection of features that I would
have liked the package to have.

Previously, if some subtree of an object that was to be populated
was not zero/nil, we would terminate the population of that subtree
then and there. However this makes the package hard to use for very
targeted dependencies that only occur at the bottom of the dependency
tree (e.g: a network client or database layer).
With this patch, the injection does move downwards and populate the
fields that it can (see inject_test.go::TestNonEmptyInterfaceTraversal for example).

Furthermore, we can now have constructors populate our structs with
default values, that will then be overridden by the injection
mechanism if the tag inject:"override" is present on the field.
This means that we can now use sane defaults when creating our objects
that we can inject mocks for during unit tests.

// A is our bogus wrapper of the http.Client
type A struct {
    Client *http.Client `inject:"override"`
}

// NewA returns a fully functional A, with a non-nil http client.
func NewA() *A {
    return &A{
        Client: &http.Client{}
    }
}

var testClient *http.Client

func TestMain(m *testing.M)
    testClient = NewTestClient()
    os.Exit(m.Run())
}

func TestWithBogusClient(t *testing.T) {
    a := NewA()
    inject.Populate(a, testClient)

    // Here the injector should have overridden the default http client
    // inside A to use the bogus client.
    a.Do(...)
}

The last feature that I was looking for was the traversal of non-nil
interfaces. For instance:

// I is our bogus interface. It has no functions for the sake of
// brevity.
type I interface {}

// A implements that bogus interface
type A struct {
    Client *http.Client `inject:""`
}

// Nested contains an I which does not need to be injected. However,
// the I (which will be of type A at runtime) needs to be traversed so that
// we can inject the right *http.Client.
type Nested struct {
    Iface I
}

func main() {
    n := &Nested{
        Iface: &A{},
    }
    specialclient := &http.Client{}
    inject.Populate(n, specialclient)
}

This is useful in bigger projects when you have many subcomponents that
use an *http.Client or sub-interface of that, and need to all have that
client mocked during unit tests.

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[emanuel-v-r]

Created this repo https://github.com/imaramos/goject with the purpose of extend this libary due to the lack of maintainance.
The only feature added was the described in this pull request #21.
The rest is only renaming.