Added package watcher to allow gin to listen to changes in packages

[theonejb]

The package watcher looks at the package in the current working directory,
and parses the source code to find out the imports. It then figures out
which of those imports are outside of the GOROOT (and thus not part of the stdlib),
and uses fsnotify.v1 to listen to changes to files in all those packages.

This allows gin to; for example, reload the web server providing a web API to another
library that is not a subpackage of the web server.

[theonejb]

@codegangsta First of all, gin is an amazing package that has been very useful to be over the past few days while developing a web app in Go. However, I missed having one feature from Django/Python, which is that the server will reload automatically even when an external package that the app depends on is changed. Thus; for instance, I could change the code for a third party library, and the Django server would reload automatically.

I really think this can be useful to gin as well. Also, this is my first pull request ever, so I'm not sure of the right way to go about contributing to any open source project. Please let me know what the next steps in this might be, and if you'll consider merging this in.