Added subcommand which takes a path to an existing binary, starts it,
and begins a debug session.

The shortening of file paths is purely a terminal UI concern. Move that
code to the presentation layer, and remove from the service layer.

This renaming avoids confusion with the 'run' subcommand in the go tool.
The 'run' subcommand in Delve is now deprecated. It is still there,
however simply prints a deprecation notice and exits.

Fixes #201. Use to connect to a running headless server

Without arguments, `list` displays source around the current context.  A
linespec argument can be taken to display source around that location.

Fixes #58

We're not dealing with a debugged process having its own controlling
terminal at this point, so no need to make the new process a session
leader. Simply making the process a group leader will suffice for our
purposes at the moment.

Fixes a bug on OSX where, if the debugged process spawned a child, when
that process received a SIGCHLD it would cause Delve to hang.

Fixes #197

We don't care, at the process level, whether or not we're single
stepping. That state is really only relevant at the thread level.

There's no reason to hang a bunch of commands off of the `info` command.
Promoted all commands to be top level.

Fixes a code path where stacktrace returns < 2 locations and
thread.ReturnAddress would panic. Now returns an error.

This fix helps avoid infinite recursion.

Original implementation was passing the wrong argument.

Breakpoints, tracepoints, etc.. take a location spec as input. This
patch improves the expressiveness of that API. It allows:

* Breakpoint at line
* Breakpoint at function (handling package / receiver smoothing)
* Breakpoint at address
* Breakpoint at file:line
* Setting breakpoint based off regexp

When running `dlv help trace` the help output would display:

dlv trace [regexp] [flags] [flags]

This patch removes the [flags] duplication.

Fixes #188