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.

Lift requirement that releaseParams() be called before tailCall() or allowCancellation() -- this is no longer necessary given the protocol simplification that allowed cap descriptors to be interpreted immediately upon receipt.

Hack to make it safe to read capabilities from default values (returning broken caps) without introducing a dependency from libcapnp on libcapnp-rpc.