Detect reinstall of current slug, and exit unless forced

This change should detect if the slug being installed is already the 'current' slug on the host, and will skip the deploy (unless 'force' was specified).

Test results:
```sh
$ slugforge deploy tag noop_test g-tapinabox.tapjoy.net --project tapjoyserver --verbose
      deploy  deploying slug tapjoyserver/tapjoyserver-20140804150120-9b4a9d90e6.slug
...
Installing package to '/opt/apps/tapjoyserver/releases/tapjoyserver-20140804150120-9b4a9d90e6'
'current' symlink points to slug: tapjoyserver-20140804144255-9b4a9d90e6
Installing as user webuser
Unpacking payload . . .
Symlinked '/opt/apps/tapjoyserver/releases/tapjoyserver-20140804150120-9b4a9d90e6' to '/opt/apps/tapjoyserver/current'
Running post install script
...
Installation complete.
SSH_COMMAND_EXIT_CODE=0
...
Deployed tapjoyserver-20140804150120-9b4a9d90e6.slug to 1 of 1 hosts in 29 seconds
```
```sh
$ slugforge deploy tag noop_test g-tapinabox.tapjoy.net --project tapjoyserver --verbose
      deploy  deploying slug tapjoyserver/tapjoyserver-20140804150120-9b4a9d90e6.slug
...
Installing package to '/opt/apps/tapjoyserver/releases/tapjoyserver-20140804150120-9b4a9d90e6'
'current' symlink points to slug: tapjoyserver-20140804150120-9b4a9d90e6
This slug is already installed in 'current'. Specify -f to force reinstall. Exiting.
SSH_COMMAND_EXIT_CODE=0
...
Deployed tapjoyserver-20140804150120-9b4a9d90e6.slug to 1 of 1 hosts in 2 seconds
```

@Tapjoy/opsautomation