Bump version to 2.0-rc

Merge pull request dotnet#23 from AArnott/sequence

 Add IBufferWriter<byte>.AsStream() extension method

Merge pull request dotnet#22 from AArnott/ReadOnlySequenceAsStream

Add ReadOnlySequence<byte>.AsStream() extension method

Distribute extension methods across more classes

Add StreamJsonRpc perf test

Merge pull request dotnet#14 from AArnott/APIChanges

Auto-close channel, update docs

Merge pull request dotnet#13 from AArnott/APIChanges

Offer ephemeral/anonymous channels and pipe I/O

Significant changes and remaining errors

Move channelCount in test to class constant

Fix appveyor build