Better coverage script

- added coveralls integration
- more efficient script logic
- simplified usage

Signed-off-by: Olivier Gambier <viapanda@gmail.com>

script/coverage

@@ -1,17 +1,10 @@
 #!/bin/bash
-
 set -e
+
 docker build -t docker-machine .
 
-if [[ "$1" == "serve" ]]; then
-    SERVE=yes
-else
-    SERVE=no
-fi
-
 docker run -it \
-    -e IN_CONTAINER=yes \
-    -e SERVE=${SERVE} \
+    -e BUILDTAGS=${BUILDTAGS} \
     -p 8000:8000 \
     --rm docker-machine \
-    ./script/generate-coverage
+    ./script/generate-coverage serve /go/src/github.com/docker/machine