Feature/aws key

When I build the gem from the Github source and try to run it using `build-ubuntu-ami`
I get the following error message:
```
~/code/build-ubuntu-ami[master]% build-ubuntu-ami examples/custom.sh   
Configuration:
  region: us-east-1
  flavor: m1.small
  brand: My
  size: 20
  codename: lucid
  key_name: default
  group: default
  arch: amd64
  canonical ami: ami-c7b202ae
  kernel: aki-427d952b
  description: My-lucid-amd64-20121106-2113
Launching server...
/Users/michael/.rvm/gems/ruby-1.9.3-p194/gems/fog-1.7.0/lib/fog/core/service.rb:208:in `validate_options': Missing required arguments: aws_access_key_id, aws_secret_access_key (ArgumentError)
	from /Users/michael/.rvm/gems/ruby-1.9.3-p194/gems/fog-1.7.0/lib/fog/core/service.rb:58:in `new'
	from /Users/michael/.rvm/gems/ruby-1.9.3-p194/gems/fog-1.7.0/lib/fog/compute.rb:15:in `new'
	from /Users/michael/.rvm/gems/ruby-1.9.3-p194/gems/fog-1.7.0/lib/fog/compute.rb:5:in `[]'
	from /Users/michael/.rvm/gems/ruby-1.9.3-p194/gems/build-ubuntu-ami-0.1.0/lib/build_ubuntu_ami.rb:93:in `launch_server!'
	from /Users/michael/.rvm/gems/ruby-1.9.3-p194/gems/build-ubuntu-ami-0.1.0/lib/build_ubuntu_ami.rb:137:in `build!'
	from /Users/michael/.rvm/gems/ruby-1.9.3-p194/gems/build-ubuntu-ami-0.1.0/bin/build-ubuntu-ami:32:in `<top (required)>'
	from /Users/michael/.rvm/gems/ruby-1.9.3-p194/bin/build-ubuntu-ami:19:in `load'
	from /Users/michael/.rvm/gems/ruby-1.9.3-p194/bin/build-ubuntu-ami:19:in `<main>'
	from /Users/michael/.rvm/gems/ruby-1.9.3-p194/bin/ruby_noexec_wrapper:14:in `eval'
	from /Users/michael/.rvm/gems/ruby-1.9.3-p194/bin/ruby_noexec_wrapper:14:in `<main>'
```

I figured that this is due to the fact that `aws_access_key_id` and `aws_secret_access_key` are never specified in the script. I therefore added the command line options `---aws_access_key_id` and `--aws_secret_access_key` so that these values can be passed into the library at runtime like this:

```
build-ubuntu-ami --aws_access_key_id=MY_AWS_ACCESS_KEY_ID --aws_secrect_access_key=MY_AWS_SECRET_ACCESS_KEY my_script.sh
```