Add Podspec

Notes: I've verified the podspec by creating a test app with a Podfile and doing `pod install`. Podfile:

```ruby
source 'https://github.com/CocoaPods/Specs.git'

platform :ios, '7.0'

pod 'NBClient', :git => 'https://github.com/3dna/nationbuilder-ios.git', :branch => 'peng/podspec'
# Change back when pod is published.
# pod 'NBClient', '~> 0.1.0'
```