Add missing framework dependecies, mark as static library and set up lib...

Fixes some missing framework dependencies and correctly specifies that this is a static library. Tested locally and appears to be working now.

```
$ pod spec lint GoogleAds-IMA-iOS-SDK.podspec

 -> GoogleAds-IMA-iOS-SDK (1.0b2)

Analyzed 1 podspec.

GoogleAds-IMA-iOS-SDK.podspec passed validation.
```

Lint looks good.