ios: Add Cocoapods support (#431)

This adds a new `EnvoyMobile.podspec` (unfortunately, "Envoy" is already taken). On `pod install` Cocoapods will download the latest prebuilt framework at /releases unless a specific version is defined in the Podfile.

Once this is merged, we'll need to publish our podspec to the Cocoapods trunk. Moving forward, we'll also need to update the version listed in the podspec during the release process.

Description: Adds Cocoapods support for distributing the prebuilt Envoy.framework
Risk Level: Low
Testing: Clone the [test project](https://github.com/cwalo/EnvoyTest) and with this branch checked out locally, run `pod install`. Be sure the two repos sit in the same parent directory (or update the podspec path in the Podfile).
Docs Changes: Yes
Release Notes: No
Fixes: [#125](envoyproxy/envoy-mobile#125)

Signed-off-by: JP Simard <jp@jpsim.com>

mobile/EnvoyMobile.podspec

@@ -0,0 +1,13 @@
+Pod::Spec.new do |s|  
+    s.name              = 'EnvoyMobile'
+    s.version           = '0.1.1'
+    s.author            = 'Lyft, Inc.'
+    s.summary           = 'Client networking libraries based on the Envoy project'
+    s.homepage          = 'https://envoy-mobile.github.io'
+    s.documentation_url = 'https://envoy-mobile.github.io/docs/envoy-mobile/latest/index.html'
+    s.social_media_url  = 'https://twitter.com/EnvoyProxy'
+    s.license           = { :type => 'Apache-2.0', :file => 'LICENSE' }
+    s.platform          = :ios
+    s.source            = { :http => "https://github.com/lyft/envoy-mobile/releases/download/v#{s.version}/envoy-ios-mac-v#{s.version}.zip" }
+    s.ios.vendored_frameworks = 'Envoy.framework'
+end  

mobile/docs/root/start/building/building.rst

@@ -87,3 +87,12 @@ example of how this artifact may be used.
 For a demo of a working app using this artifact, see the :ref:`hello_world` example.
 
 .. _releases: https://github.com/lyft/envoy-mobile/releases
+
+---------
+CocoaPods
+---------
+
+If you use CocoaPods, you can add the following to your Podfile to use the latest version of the prebuilt framework.
+
+``pod 'EnvoyMobile'``
+