calculator.capnp style adjusting

p.s. thanks @cstrahan for vim syntax highlighting

Delete snappy code.  It relied on an unsubmitted patch to Snappy's interface and it turns out LZ4 is better anyway so we'll probably integrate with that instead, eventually.

Upgrade gtest to 1.7.0

doc: Fix documentation pseudocode to match description.

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.