This supersedes the `capnp encode` and `capnp decode` commands. It didn't make sense to add JSON to those commands since it was unclear if JSON should be thought of as the "encoded" or "decoded" format. `convert` allows mapping anything to anything.

This command is also useful for, say, converting unpacked format to packed format or vice versa, which can now be done without a schema.

Allow an ExceptionCallback to control how new threads' ExceptionCallbacks are initialized.

Fix minor typo: varibale -> variable

Clarify RPC spec documentation based on mail thread

Kenton rightly pointed out that this is an unnecessary round-trip.

Update README.md

protobuf link update

Currently only implemented for Linux (when using futexes), since that's what I use. Will expand to other platforms later.

synchrozines behavior with autoconf build

https://github.com/capnproto/capnproto/issues/515

https://groups.google.com/d/topic/capnproto/_1kBtRSC51s/discussion

Doc: Enforce comment-after-code rule

Install capnp-json.pc

While comments should appear after the code, some examples violate the convention. This commit enforces the rule.

Minor: fix typo in `capnp help encode` documentation

Add versioning to the libraries in the CMake build

The Autotools and CMake builds are almost the same, except for
the libraries not being generated as versioned libraries in CMake.

Fixing this lets Linux CMake builds be on the same level as Autotools ones.

Add cmake_minimum_required to root CMakeLists.txt

There was already a minimum version specified in the actual CMakeLists
file in the C++ directory, but since the parent file does not have it
you get a warning if you build in the idiomatic cmake fashion:

    mkdir build
    cd build
    cmake .. # <---- warning here because no minimum version specified

I used 3.1 as min version because that is the same as specified in child
file.

This reverts commit b50f6598.