Add -wait parameter to debounce watched events

[gigablah]

Addresses the following issues: #42, #43, #57, #103, #136

This is a rewrite of generateFromEvents that makes the following changes:

• Only configs that have watch = true will be generated upon container events.
• All configs will be regenerated when the events channel is reattached, rather than upon docker client re-initialization. (avoids missing out on events in between)
• Each config with watch now has its own separate channel. This is to enable individual debouncing parameters, specified with the wait parameter. Syntax is min:max, e.g. 500ms:2s. This feature is referenced from hashicorp/envconsul

I've also added an integration test for generator.go that verifies the wait parameter :)

[gigablah]

A few more changes:

• I made generateAtInterval and generateFromEvents aware of termination signals as well (replaced an apparently unused quit channel in generateAtInterval in the process)
• Made the docker daemon reconnection logic more robust, so now it will survive a docker restart
• Also added an internal retry flag for the generator which determines if it will attempt to reconnect if the events channel is closed, mainly for testing purposes.

[jgallen23]

this is awesome! This solves all my issues and would let me safely run docker-gen in prod and give me nearly zero-downtime deployments

[jwilder]

Nice @gigablah! Thanks!