Adds descriptive error when failing to make concrete (#51)

If using lazy loading and the call to a resolver when making a concrete
fails, that error wasn't wrapped and caused confusion since we didn't
know which abstraction we were attempting to satisfy.

Wrapping error so we know that:
  a) Error occurred during resolve call.
  b) What abstraction we were trying to resolve.

This should make it easier to debug these failures in lazy loading situations.

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Adds lazy loading support to binding methods (#44)

* Adds lazy loading support to binding methods

Inspired from work by @Place1. Adds new Lazy methods that can optionally
be called to lazy bind a resolver to an abstract. The resolver will not be
called until the first time a resolve call is made.

A strong effort was made to not cause regressions or otherwise change the
behavior of the existing API. Minor changes were made to how validation
occurs and in some situations a more verbose error can be presented earlier
in a binding process.

* Improves test coverage

nil handling for Call() (#40)

* nil handling for Call()

* added a testcase

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fix issue #37 (#38)

Fix issue #37

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flat structure

flat structure

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