See #131, #191.

- The installer is initialized with a resolver. The resolver is responsible of
  indicating which specs must be installed/reinstalled.
- It was introduced a slight change in the format of the Podfile.lock.
- The specification set was simplified to receive and handle Pod::Dependency
  instead of Pod::Specification. With this change it also appears to be more
  robust.

A this stage it appears to be working. However the support, for external and
head dependencies is weak.