Fix bug where receiving EOF on the RPC connection could cause RpcConnectionState to go into an infinite promise loop.  Strange that this did not cause trouble earlier.  Also some minor tweaks.

Fixes #67.  Also, prevent people from accidentally making MessageBuilder subclasses copyable/movable as brought up in issue #64.

warning: 'constexpr' non-static member function will not be implicitly
'const' in C++1y; add 'const' to avoid a change in behavior
[-Wconstexpr-not-const]

Update test script for XCode 5, make sure -stdlib gets in pkg-config configs, use pkg-config in test script, and build RPC tests in test script.

Eliminate the concept of imbuing messages in favor of the simpler concept of setting a cap table directly on MessageReader / getting one from MessageBuilder.  This eliminates capability-context entirely.  This was made possible by the earlier change which moved capabilities to a separate table rather than storing CapDescriptors inline, but I didn't realize it at the time.

Define additional type aliases AnswerId and ImportId.  In the RPC protocol, always use the type corresponding to the sender's point of view.  In the implementation, always use the type corresponding to the local point of view.