Implementation of multiline syntax for Dockerfile

This PR implments #1799.  

Passes all tests, with what appear to be harmless (?) warnings:

```
$ time sudo GOPATH=/home/jason/work/go /home/jason/bin/go test
2013/09/09 17:03:22 Listening for HTTP on 127.0.0.1:4270 (tcp)
2013/09/09 17:03:25 WARNING: Your kernel does not support swap limit capabilities. Limitation discarded.
2013/09/09 17:03:25 WARNING: Your kernel does not support swap limit capabilities. Limitation discarded.
2013/09/09 17:03:28 Container b1834cfb30eace958b61fe28260cc0fc2b45481b3594b2fbcca3c9b86f9d534e failed to exit within 1 seconds of SIGTERM - using the force
2013/09/09 17:03:31 Container d897754a96f30f067c975a28a330534574572c8f1b2256e1f223a24c9a618e93 failed to exit within 1 seconds of SIGTERM - using the force
2013/09/09 17:03:46 error killing container 57ef82d41eff720215e5a534c6d992634cfb34247243414a346863e8b1a17f7b (lxc-kill: failed to get the init pid
, exit status 255)
2013/09/09 17:03:46 error killing container d5e6fb3fb8e8c350ba392604fd87dce295667eb5598a2c3ca123b976d1594ff3 (lxc-kill: failed to get the init pid
, exit status 255)
2013/09/09 17:03:47 error killing container 11b9887bc338c05abcade0a2878c26a2116ef334baaa7e40db5843233a5928c4 (lxc-kill: failed to get the init pid
, exit status 255)
2013/09/09 17:03:58 WARNING: Your kernel does not support swap limit capabilities. Limitation discarded.
PASS
ok  	github.com/dotcloud/docker	56.965s

real	0m58.942s
user	0m6.020s
sys	0m9.624s
```