Support for gdm (go dependency management)

Hi,

This is following lp:1748202.
A lot of Go projects use gdm to manage dependencies. (example: https://github.com/influxdata/telegraf)
Snapcraft only offers the `go` plugin, which pulls all subpackages in their latest version (e.g. github.com/influxdata/telegraf/...) or the `godeps` plugin, using github.com/rogpeppe/godeps, which uses a different input file format.

Example when trying to use the godeps plugin:
snapcraft.yaml:
parts:
  telegraf:
    source: .
    plugin: godeps
    go-importpath: github.com/influxdata/telegraf
    godeps-file: Godeps

Output:
sajoupa@portable:~/dev/telegraf{make-it-a-snap}$ snapcraft
Preparing to pull telegraf
Pulling telegraf
Fetching godeps...
go get github.com/rogpeppe/godeps
Obtaining project dependencies...
godeps -t -u /home/sajoupa/dev/telegraf/parts/telegraf/src/Godeps
godeps: cannot parse "/home/sajoupa/dev/telegraf/parts/telegraf/src/Godeps": cannot parse "collectd.org 2ce144541b8903101fb8f1483cc0497a68798122": expected 4 tab-separated fields, got 1
Traceback (most recent call last):
[...]

Using the go plugin: see lp:1748202

Thanks,
Laurent