First draft, Still WIP (TODO = Add specs + add `pod cache clean` command)

[Create] Allow passing additional args to the configure script

[Validator] Explain why linting failed

Add a pre-install hook

Make updating specs repos its own UI section

[Cache] Remove cache when CocoaPods version changes

Symlink vendored framework headers into the public headers dir

Fixes #3161

[Travis] Only build master and stable branches

Flush stdin/stderr and wait a bit in `executable`.

This should make it unlikely that we don't catch the full output of
commands we are executing.

[Cache] Recognize incomplete cache for released pods

Even when cached_spec isn't used further with local podspecs, it still must be read from file to verify that it was correctly written before, because that also indicates that the cached pod is valid.