add rake tasks for web hooks management

these rake tasks allows add/remove web hooks for multiple projects at once. 

### Examples

Adding a web hook for **ALL** projects:

    RAILS_ENV=production bundle exec rake gitlab:web_hook:add URL="http://example.com/hook"

Adding a web hook for projects in a given **NAMESPACE**:

    RAILS_ENV=production bundle exec rake gitlab:web_hook:add URL="http://example.com/hook" NAMESPACE=acme

Similarly, it's possible remove a web hook from **ALL** projects using:

    RAILS_ENV=production bundle exec rake gitlab:web_hook:rm URL="http://example.com/hook"

Removing web hooks with **URL** for projects in a given **NAMESPACE**:

    RAILS_ENV=production bundle exec rake gitlab:web_hook:rm URL="http://example.com/hook" NAMESPACE=acme


**Extra:**

Listing **ALL** web hooks:

    RAILS_ENV=production bundle exec rake gitlab:web_hook:list

Listing the web hooks from projects in a given **NAMESPACE**:

    RAILS_ENV=production bundle exec rake gitlab:web_hook:list NAMESPACE=/

> Note: `/` is the global namespace.