Add a 'Continue' function.
Add a call to send a signal to the tracee.

Notably this can be used to stop the tracee (via syscall.SIGSTOP).
drop lock and add a `Close' call.

this makes the user responsible for cleanup, instead of it
happening asynchronously.
rename TraceeExited to ErrExited.

To placate golint.
Merge pull request #3 from tfogal/pr

updated PR: fix bug, add command, drop lock.