Add Octokit PCL and all relevant plumbing

Add Octokit-PCL and ConcurrentCache which is used by the PCL in lieu of
the ConcurrentDictionary, which is not available on Windows Phone 8.
Adding Tests for the PCL project

Tests for pcl project, looks like this is sticking to the established
convention :)
Make internals visible to.

I forgot about this. My mistake.
Adding Documentation

Update Readme and add a small description and correctly attribute the
ConcurrentCache.
Reference to Microsoft.Bcl.Build for PCL-Tests

Tackling this warning:

All projects referencing Octokit-Pcl.csproj must install nuget package
Microsoft.Bcl.Build. For more information, see
http://go.microsoft.com/fwlink/?LinkID=317569. Octokit.Tests-Pcl
Revert "Reference to Microsoft.Bcl.Build for PCL-Tests"

This reverts commit 8bbf2bf6940d8a7b1d624bad6671ee18daa761be.
Remove Reference to BCL, abstract HttpClient

I'll let the experts figure out Microsoft.Bcl.Build and the ensuing
warnings. :thumbsup:

Wrapped ProductHeaderValue class, hope this is right. Made some changes
to ensure the project uses the new, wrapped, ProductHeaderValue.
Partial done on nuget.

Working on nuget, this is where I am really weak.
yes the bcl
merging master into this branch, i think
Adding Gist feature to PCL.
Starting in on fixing the nuspec

Let's see how this works and handling dependency versioning
Renaming 'PCL' to 'Portable' :tada:

Gotta portable nuget package I am finishing testing and figured this
would be consistent. :sparkle:
stubbed prototype method to support comparing two commits
added reactive client and tidied up xml-docs
added unit tests
moved function out to separate client
added proper reactive client
merging again - needing guidance on nuget
Merge branch 'lets-make-a-pcl' of https://github.com/trsneed/octokit.net into lets-make-a-pcl