WIP: Migrating to using Portable Class Libraries

I initially had grand plans to have a .NET 4 build alongside this, but it was much :rage1: 

- [x] switch the Octokit and Octokit.Reactive projects to target Profile78
- [ ] introduce Microsoft.Bcl.Async, Microsoft.Bcl.Immutable and Microsoft.Net.Http dependencies
- [ ] resolve impacted test (see stack trace below):
- [ ] update `CreatePackages` step to ensure paths are correct and it all Just Works
- [ ] ???

```
Octokit.Tests.Integration.MiscellaneousClientTests+TheGetEmojisMethod.GetsAllTheEmojis [FAIL]
   System.ArgumentException : Incorrect number of arguments supplied for call to method 'Void set_Item(System.String, System.String)'
   Stack Trace:
      at System.Linq.Expressions.Expression.ValidateArgumentCount(MethodBase method, ExpressionType nodeKind, Int32 count, ParameterInfo[] pis)
      at System.Linq.Expressions.Expression.ValidateArgumentTypes(MethodBase method, ExpressionType nodeKind, ReadOnlyCollection`1& arguments)
      at System.Linq.Expressions.Expression.Call(Expression instance, MethodInfo method, IEnumerable`1 arguments)
      c:\Users\brendanforster\Documents\GitHub\octokit.net\Octokit\SimpleJson.cs(1934,0): at Octokit.Internal.ReflectionUtils.GetSetMethodByExpression(PropertyInfo propertyInfo)
      c:\Users\brendanforster\Documents\GitHub\octokit.net\Octokit\SimpleJson.cs(1263,0): at Octokit.PocoJsonSerializerStrategy.SetterValueFactory(Type type)
      c:\Users\brendanforster\Documents\GitHub\octokit.net\Octokit\SimpleJson.cs(1991,0): at Octokit.Internal.ReflectionUtils.ThreadSafeDictionary`2.AddValue(TKey key)
      c:\Users\brendanforster\Documents\GitHub\octokit.net\Octokit\SimpleJson.cs(1982,0): at Octokit.Internal.ReflectionUtils.ThreadSafeDictionary`2.Get(TKey key)
      c:\Users\brendanforster\Documents\GitHub\octokit.net\Octokit\SimpleJson.cs(1369,0): at Octokit.PocoJsonSerializerStrategy.DeserializeObject(Object value, Type type)
      c:\Users\brendanforster\Documents\GitHub\octokit.net\Octokit\Http\SimpleJsonSerializer.cs(93,0): at Octokit.Internal.SimpleJsonSerializer.GitHubSerializerStrategy.DeserializeObject(Object value, Type type)
      c:\Users\brendanforster\Documents\GitHub\octokit.net\Octokit\SimpleJson.cs(571,0): at Octokit.SimpleJson.DeserializeObject[T](String json, IJsonSerializerStrategy jsonSerializerStrategy)
      c:\Users\brendanforster\Documents\GitHub\octokit.net\Octokit\Http\JsonHttpPipeline.cs(47,0): at Octokit.Internal.JsonHttpPipeline.DeserializeResponse[T](IResponse`1 response)
      c:\Users\brendanforster\Documents\GitHub\octokit.net\Octokit\Http\Connection.cs(304,0): at Octokit.Connection.<Run>d__c`1.MoveNext()
   --- End of stack trace from previous location where exception was thrown ---
      at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
      at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
      c:\Users\brendanforster\Documents\GitHub\octokit.net\Octokit\Clients\MiscellaneousClient.cs(40,0): at Octokit.MiscellaneousClient.<GetEmojis>d__4.MoveNext()
   --- End of stack trace from previous location where exception was thrown ---
      at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
      at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
      c:\Users\brendanforster\Documents\GitHub\octokit.net\Octokit.Tests.Integration\Clients\MiscellaneousClientTests.cs(19,0): at Octokit.Tests.Integration.MiscellaneousClientTests.TheGetEmojisMethod<GetsAllTheEmojis>d__1.MoveNext()

71 total, 1 failed, 3 skipped, took 265.332 seconds
```