Add `PIDFile` for managing a Sanford process PID file

This adds a helper class for managing a Sanford process PID file.
When a Sanford process is started, it writes its PID file to a
configured path. This is used to control the Sanford process when
using the CLI. When a stop, halt or restart command is given, the
PID file will be read and a TERM, INT or USR2 signal will be sent
to the process. This logic will be used by both the building and
running a Sanford process set of logic and the sending signals
to a Sanford process set of logic.
Add `Process` logic, for running a Sanford server process

This adds the logic related to running a Sanford server process.
This logic replaces the `Manager` logic and will be used by the
CLI. The `Process` class takes a server and sets up the current
ruby process for it when run. This involves optionally daemonizing
the process, managing a PID file, and starting the server. The
process also traps signals for controlling the process after its
running. The server returns a thread when its started and this is
joined to the main thread, so the process will continue forever
until it receives a signal. It is setup to receive `TERM` which
will stop it gracefully, `INT` which will halt it immediately (not
graceful), and `USR2` which will perform a hot restart. To perform
the hot restart, the process runs a `Kernel.exec` which replaces
the current process with the new process defined by the args passed
to the `exec`. The TCP server is only paused when this happens, so
its still accepting connections, just not processing them.

This fixes some bad practices in the old `Manager` logic related
to the trapping of signals. The server commands for `stop`, `halt`
and `pause` do not perform any of the logic for actually stopping
the server. They proxy DatTCP which simply writes the command to
a `IO.pipe` and then exits. The original thread running the server
then gets this and handles shutting down the server as needed.
This is a better practice than doing all the processing in the
actual signal traps. I ran into problems with the old logic that
are not present with this implementation.