Attachment support for custom filename and or mimetype

Fixes #15
Fixed HTTPClient::setApiKey, wasn't working correctly

The HTTP header with the API wasn't updated. Using setters to prevent bugs like these.
Added timeout parameter to the config and HTTPClient

this parameter gives you the ability to specify after how many seconds sending a message may take. Default is 5 seconds.

Fixes #16
Updated the Readme

- Add timeout parameter to the config example
- Show that you can set a filename and/or mimetype of an attachment in the example code
Added scalarNode to support timeout parameter in configurationfile

forgot to add it to the Configuration class.
Also add timeout config param to MZPostmarkExtension class
Merge pull request #17 from wrep/feature/attachment-improvements

Some attachment improvements and change to curl timeout.
Support for batch sending
Updated readme with batch example
Merge pull request #18 from ruudk/feature-batch

Support for batch sending