Add rdoc to development dependencies and merge with master

I found that without merging `master` into this branch was causing many errors and that we needed to add rdoc as a development dependency to even run with 1.8.7. 

Otherwise, here's the current state: 1 failure across versions 1.9 versions, and an extra nastier failure (test email not delivered) with 1.8.7.

### Ruby 1.9.3-p545, Ruby 1.9.2-p320 and Ruby 1.8.7-p374

```
1) Gmail client (Plain) instance shouldn't raise error even though GMail account is invalid
     Failure/Error: lambda {
       expected no Exception, got #<ArgumentError: wrong number of arguments (1 for 0)>
```

### Ruby 1.8.7-p374

```
2) Gmail client (Plain) instance should deliver inline composed email
     Failure/Error: client.deliver do
       expected false to be true

Finished in 27.5 seconds
43 examples, 2 failures, 12 pending

Failed examples:

rspec ./spec/client_spec.rb:53 # Gmail client (Plain) instance shouldn't raise error even though GMail account is invalid
rspec ./spec/client_spec.rb:102 # Gmail client (Plain) instance should deliver inline composed email
rake aborted!
/Users/alex/.rvm/rubies/ruby-1.8.7-p374/bin/ruby -S rspec ./spec/client_spec.rb ./spec/gmail_spec.rb ./spec/mailbox_spec.rb ./spec/message_spec.rb failed
/Users/alex/.rvm/gems/ruby-1.8.7-p374/gems/rspec-core-2.10.1/lib/rspec/core/rake_task.rb:137:in `initialize'
/Users/alex/.rvm/gems/ruby-1.8.7-p374/gems/rspec-core-2.10.1/lib/rspec/core/rake_task.rb:127:in `send'
/Users/alex/.rvm/gems/ruby-1.8.7-p374/gems/rspec-core-2.10.1/lib/rspec/core/rake_task.rb:127:in `initialize'
Tasks: TOP => default => spec
(See full trace by running task with --trace)
```