implement git deploy method

This commit implements the git deploy method. This is basically a copy of the
Heroku deploy method except it uses a target instead of an app. The target can
be anything that git accepts as a url.

An example of deploy using git can be:

    deploy:
      git:
        target: git@example.com:myapp.git

pkg/plugin/deploy/git.go

@@ -1 +1,38 @@
 package deploy
+
+import (
+	"fmt"
+	"github.com/drone/drone/pkg/build/buildfile"
+)
+
+type Git struct {
+	Target string `yaml:"target,omitempty"`
+	Force  bool   `yaml:"force,omitempty"`
+	Branch string `yaml:"branch,omitempty"`
+}
+
+func (g *Git) Write(f *buildfile.Buildfile) {
+	// get the current commit hash
+	f.WriteCmdSilent("COMMIT=$(git rev-parse HEAD)")
+
+	// set the git user and email based on the individual
+	// that made the commit.
+	f.WriteCmdSilent("git config --global user.name $(git --no-pager log -1 --pretty=format:'%an')")
+	f.WriteCmdSilent("git config --global user.email $(git --no-pager log -1 --pretty=format:'%ae')")
+
+	// add target as a git remote
+	f.WriteCmd(fmt.Sprintf("git remote add deploy %s", g.Target))
+
+	switch g.Force {
+	case true:
+		// this is useful when the there are artifacts generated
+		// by the build script, such as less files converted to css,
+		// that need to be deployed to git remote.
+		f.WriteCmd(fmt.Sprintf("git add -A"))
+		f.WriteCmd(fmt.Sprintf("git commit -m 'add build artifacts'"))
+		f.WriteCmd(fmt.Sprintf("git push deploy $COMMIT:master --force"))
+	case false:
+		// otherwise we just do a standard git push
+		f.WriteCmd(fmt.Sprintf("git push deploy $COMMIT:master"))
+	}
+}