Support for incoming requests from TextMarks

TextMarks allows a web application to receive notifications when subscribed users send text messages with the user's keyword.

To enable this support, a user should log into his TextMarks account, then edit a keyword and then go to Settings -> API.

The attached patch implements this support in the module in the following way:

• The URL http://yourdrupalwebsite.com/textmarks is used as the callback URL.
• Support for a private token and its validation.
• Support for any module to implement hook_textmarks() and perform any tasks needed with the information received from TextMarks, as well as returning a text that will be used as the reply to TextMarks.