fix purge_all, purge_by_key and details at Fastly::Service

Hi, i just tried to automatically purge Fastly when we deploy the app in to production. But the ruby clients halted with the following error. Could you merge the change and release new version? Thanks -K

```text
undefined local variable or method `client' for #<Fastly::Service:0x007ff982c11d70>
/Users/kzk/.rbenv/versions/2.0.0-p0/lib/ruby/gems/2.0.0/gems/fastly-1.00/lib/fastly/service.rb:62:in `purge_all'
/Users/kzk/repos/td-docs/Rakefile:34:in `block (2 levels) in <top (required)>'
/Users/kzk/repos/td-docs/Rakefile:33:in `each'
/Users/kzk/repos/td-docs/Rakefile:33:in `block in <top (required)>'
/Users/kzk/.rbenv/versions/2.0.0-p0/lib/ruby/gems/2.0.0/gems/rake-0.9.2.2/lib/rake/task.rb:205:in `call'
/Users/kzk/.rbenv/versions/2.0.0-p0/lib/ruby/gems/2.0.0/gems/rake-0.9.2.2/lib/rake/task.rb:205:in `block in execute'
/Users/kzk/.rbenv/versions/2.0.0-p0/lib/ruby/gems/2.0.0/gems/rake-0.9.2.2/lib/rake/task.rb:200:in `each'
/Users/kzk/.rbenv/versions/2.0.0-p0/lib/ruby/gems/2.0.0/gems/rake-0.9.2.2/lib/rake/task.rb:200:in `execute'
/Users/kzk/.rbenv/versions/2.0.0-p0/lib/ruby/gems/2.0.0/gems/rake-0.9.2.2/lib/rake/task.rb:158:in `block in invoke_with_call_chain'
/Users/kzk/.rbenv/versions/2.0.0-p0/lib/ruby/2.0.0/monitor.rb:211:in `mon_synchronize'
/Users/kzk/.rbenv/versions/2.0.0-p0/lib/ruby/gems/2.0.0/gems/rake-0.9.2.2/lib/rake/task.rb:151:in `invoke_with_call_chain'
/Users/kzk/.rbenv/versions/2.0.0-p0/lib/ruby/gems/2.0.0/gems/rake-0.9.2.2/lib/rake/task.rb:144:in `invoke'
/Users/kzk/.rbenv/versions/2.0.0-p0/lib/ruby/gems/2.0.0/gems/rake-0.9.2.2/lib/rake/application.rb:116:in `invoke_task'
/Users/kzk/.rbenv/versions/2.0.0-p0/lib/ruby/gems/2.0.0/gems/rake-0.9.2.2/lib/rake/application.rb:94:in `block (2 levels) in top_level'
/Users/kzk/.rbenv/versions/2.0.0-p0/lib/ruby/gems/2.0.0/gems/rake-0.9.2.2/lib/rake/application.rb:94:in `each'
/Users/kzk/.rbenv/versions/2.0.0-p0/lib/ruby/gems/2.0.0/gems/rake-0.9.2.2/lib/rake/application.rb:94:in `block in top_level'
/Users/kzk/.rbenv/versions/2.0.0-p0/lib/ruby/gems/2.0.0/gems/rake-0.9.2.2/lib/rake/application.rb:133:in `standard_exception_handling'
/Users/kzk/.rbenv/versions/2.0.0-p0/lib/ruby/gems/2.0.0/gems/rake-0.9.2.2/lib/rake/application.rb:88:in `top_level'
/Users/kzk/.rbenv/versions/2.0.0-p0/lib/ruby/gems/2.0.0/gems/rake-0.9.2.2/lib/rake/application.rb:66:in `block in run'
/Users/kzk/.rbenv/versions/2.0.0-p0/lib/ruby/gems/2.0.0/gems/rake-0.9.2.2/lib/rake/application.rb:133:in `standard_exception_handling'
/Users/kzk/.rbenv/versions/2.0.0-p0/lib/ruby/gems/2.0.0/gems/rake-0.9.2.2/lib/rake/application.rb:63:in `run'
/Users/kzk/.rbenv/versions/2.0.0-p0/lib/ruby/gems/2.0.0/gems/rake-0.9.2.2/bin/rake:33:in `<top (required)>'
/Users/kzk/.rbenv/versions/2.0.0-p0/bin/rake:23:in `load'
/Users/kzk/.rbenv/versions/2.0.0-p0/bin/rake:23:in `<main>'
Tasks: TOP => purge_cdn
```