This PR implements and closes#137. The new `Endpoint` and `Context`
classes expose the connection establishing functionality from
`Communicator`, which now is only responsible for tying together the
bootstrapper with a context.
The largest breaking change here is that
`Communicator.connectOnSetup(...)` now returns the `Connection` wrapped
inside a `NonblockingFuture`. This is because with the way `Context` is
implemented a `Connection` is now fully initialized on construction.
Some smaller breaking API changes from this change are that
`RegisteredMemory` no longer has a `rank()` function (as there maybe no
concept of rank), and similarly `Connection` has no `remoteRank()` and
`tag()` functions. The latter are replaced by `remoteRankOf` and `tagOf`
functions in `Communicator`.
A new `EndpointConfig` class is introduced to avoid duplication of the
IB configuration parameters in the APIs of `Context` and `Communicator`.
The usual usage pattern of just passing in a `Transport` still works due
to an implicit conversion into `EndpointConfig`.
Miscellaneous changes:
-Cleans up how the PIMPL pattern is applied by making both the `Impl`
struct and the `pimpl_` pointers private for all relevant classes in the
core API.
-Enables ctest to be run from the build root directory.
