Option to Create a Drupal User from a Profile

this issue is as per discussion on civicrm-dev list
http://www.nabble.com/Option-to-Create-a-Drupal-User-from-a-Profile---Contribution-Page-tf3256510s15986.html

1. Add new setting for Profiles : "Create CMS User"

2. If this is enabled, the Profile form fieldset is "enhanced" as
follows when the form is displayed AND the visitor is NOT logged in to
Drupal:

• Add checkbox: "Create an account?"
• If checked, display fields for entering Username, Password,
  Confirm Password

(The profile must include an Email Address, or this feature is suppressed)

3. When the profile is submitted, we would invoke the same Drupal API
used when a new user registers "normally". This should mean that "post
user creation processes" configured for a given site would be
triggered (e.g. sending welcome emails, etc.)

check drupal api: http://api.drupal.org/api/HEAD/function/drupal_execute

4. Error Handling -

• We need to handle the case where a User with this Username and/or
  Email already exists. We could either return an error message and not
  submit the profile form (tell them to login, etc.). Or, we could
  bypass creating the account. If we bypass creating the account, we
  mIght need to tell them that a new user account was NOT created
  because an account already exists...and to use their existing login...