Service aliases

[pelletier]

This commit makes it possible for a given container to advertise multiple services using aliases.

For example, the following config:

SERVICE_NAME=a
SERVICE_ALIASES=b,c

will result in 3 services being registered: a, b, and c. All three pointing to the same container.

Those changes should be compatible with ports, and tags.

This should fix https://github.com/gliderlabs/registrator/issues/328

[pelletier]

@josegonzalez any chance we could get some feedback on this please? :)