Support emitting a reload signal to services

It's currently possible to send `restart` to a process based on a rule, which is very handy for background processes. Some services can accept `reload` as a custom event and use it to perform a hot restart (unicorn) or a phased restart (puma), which is preferable for foreground services.

For this to be truly useful with forking services (i.e. unicorn) we would need a means of tracking total memory, ala #31.

Reload support in upstart: http://upstart.ubuntu.com/cookbook/#reload-signal

This seems like a straight forward addition if anybody else sees a need for it. @mperham: What do you think?
I think you might be overthinking this.  According to the docs, runit supports "reload" which sends HUP as expected:

http://smarden.org/runit/sv.8.html

Let's be consistent and assume that reload == HUP.