rough draft for systemd export support

http://0pointer.de/blog/projects/systemd.html

This adds support for exporting systemd targets and services.  The
structure is based on the existing upstart support.

Quality is draft and expected to refine in the coming weeks.

One Foremanism that is not respected by these export templates is the
usual log output location, instead stdout and stderr go to syslog.

man/foreman.1.ronn

@@ -103,6 +103,8 @@ foreman currently supports the following output formats:
 
   * runit
 
+  * systemd
+
   * upstart
 
 ## INITTAB EXPORT
@@ -114,6 +116,17 @@ Will export a chunk of inittab-compatible configuration:
     EX02:4:respawn:/bin/su - example -c 'PORT=5100 bundle exec rake jobs:work >> /var/log/job-1.log 2>&1'
     # ----- end foreman example processes -----
 
+## SYSTEMD EXPORT
+
+Will create a series of systemd scripts in the location you specify. Scripts
+will be structured to make the following commands valid:
+
+  `systemctl start appname.target`
+
+  `systemctl stop appname-processname.target`
+
+  `systemctl restart appname-processname-3.service`
+
 ## UPSTART EXPORT
 
 Will create a series of upstart scripts in the location you specify. Scripts