[Outdated] Refine pod outdated to show what pod update would do

Addresses https://github.com/CocoaPods/CocoaPods/issues/2470.

Not ready to merge. Needs test coverage and likely some UI refinements.

Before:
```
The following updates are available:
- Expecta 0.2.3 -> 0.3.1
- libextobjc 0.3 -> 0.4
- OCMock 2.2.2 -> 3.1.1
- ReactiveCocoa 2.1.8 -> 2.3.1
- ReactiveViewModel 0.1.1 -> 0.2
```

After:
```
Inspecting targets to integrate
Resolving dependencies of `Podfile`
Comparing resolved specification to the sandbox manifest
The following updates are available:
- Expecta 0.2.3 -> 0.2.4 (last version 0.3.1)
- libextobjc 0.3 -> 0.3 (last version 0.4)
- OCMock 2.2.2 -> 2.2.4 (last version 3.1.1)
- ReactiveCocoa 2.1.8 -> 2.1.8 (last version 2.3.1)
- ReactiveViewModel 0.1.1 -> 0.1.1 (last version 0.2)
```