add support for explicit subdomains

this way, if you link something as foo.example, it will be reachable
from foo.example.dev, or more generally $prefix.foo.example.dev.

This is needed at my work since we have several apps that need to run on
the same domain, so it's easier to set up all the apps as
app-name.company-domain.dev.

This should be a backwards-compatible change, since arbitrary subdomains
still work in the case that was supported before.
remove some debugging logs
Added prax uninstall command

As raised by #46 there was no way to easily uninstall Prax. Until
now :)
change xip_app_name to xip_segments

for explicit subdomains
Merge pull request #47 from jayferd/feature.explicit-subdomains

add support for explicit subdomains
detect if racker fails to start
render cant_start_app if racker fails to start
at least log the error if config.ru is broken