Adds podspec

Also updates README instructions for installation. 
Do we need a `source 'https://rubygems.org'` line here?

I happen to get this without it:
```text
jeff@cyborg ~/w/Nimble-CocoaPods> bundle install
Fetching https://github.com/CocoaPods/CocoaPods.git
Fetching https://github.com/CocoaPods/Core.git
Fetching https://github.com/CocoaPods/Xcodeproj.git
Resolving dependencies...
Your Gemfile has no gem server sources. If you need gems that are not already on your
machine, add a line like this to your Gemfile:
source 'https://rubygems.org'
Could not find gem 'cocoapods-trunk (~> 0.4.0) ruby', which is required by gem 'cocoapods
(>= 0) ruby', in any of the sources.
```