Add podspec for Google IMA iOS SDK v1.0b2

Hello! This is my first podspec, so please let me know if anything looks out of place.

I copied much of the spec from the existing Google* project specs that already exist and link against existing Google downloads.

There are a couple of lint warnings, but they appear to be xcodebuild things related to the library:
```
 -> GoogleAds-IMA-iOS-SDK (1.0b2)
    - NOTE  | [xcodebuild]  warning: no rule to process file '$(PROJECT_DIR)/GoogleAds-IMA-iOS-SDK/GoogleIMASDK3/libGoogleIMA3.a' of type archive.ar for architecture armv7
    - NOTE  | [xcodebuild]  warning: no rule to process file '$(PROJECT_DIR)/GoogleAds-IMA-iOS-SDK/GoogleIMASDK3/libGoogleIMA3.a' of type archive.ar for architecture armv7s

Analyzed 1 podspec.

GoogleAds-IMA-iOS-SDK.podspec passed validation.
```

Let me know if that looks like an issue I should fix.