Add support for Bundler

The result:

```
active_mailbox (bundler)]$ bundle install
Resolving dependencies...
Using rake (0.8.7)
Using activesupport (2.0.0)
Using rmail (1.0.0)
Using tmail (1.2.3.1)
Using activemailbox (0.0.3) from source at .
Using rcov (1.0.0)
Using bundler (1.3.2)
Your bundle is complete! Use `bundle show [gemname]` to see where a bundled gem is installed.
```

```
[active_mailbox (bundler)]$ bundle exec rake -T
(in /Users/mlafeldt/devel/projects/active_mailbox)
rake clean               # Cleanup everything
rake clobber_package     # Remove package products
rake clobber_rcov        # Remove rcov products for rcov
rake clobber_rdoc        # Remove rdoc products
rake decrypt_passphrase  # Decrypt passphrase used in test configuration files
rake dist                # Create distribution (packaging & documentation)
rake distclean           # Cleanup distribution
rake encrypt_passphrase  # Encrypt passphrase for use in test configuration files
rake gem                 # Build the gem file activemailbox-0.0.3.gem
rake generate_aes_key    # Generate AES encryption key to use in main configuration file
rake package             # Build all the packages
rake rcov                # Analyze code coverage with tests
rake rdoc                # Build the rdoc HTML Files
rake repackage           # Force a rebuild of the package files
rake rerdoc              # Force a rebuild of the RDOC files
rake test                # Run tests
```