Add field type image to profile module

This patch adds a working beta version of an image field to the profile module. Admin can add image fields to the user profile in the same way other fields are added. Uploaded images are saved in the files directory. This is a working version but there is much room for improvement.

TODOs include:
-- add admin customization for image size. Admin should be able to specify maximum filesize and/or dimensions.
-- Code needs to be evaluated for conciseness and quality.
-- Display of images in the edit and profile views can be improved.
-- method _get_height_width_attributes($image_path) potentially has an equivalent in another method, and if not should possibly be moved to a module/inc where it makes more sense.

My intended use for the patch is to offer users a way to upload images to add styles to their personal blogs. I am offering a field where they can input CSS styles that get written into the header. Now they can upload images to use as background images or logos for their personal blogs.

The patch includes a couple of changes that should be refactored into another patch, which I will do in the next week. I didn't have time to do this today but I wanted to make the patch available for comments and evaluation. The changes referenced are the following:

in profile_form_profile I added t() calls to the titles of all fields.

in profile_validate_profile I changed the input parameter $edit to &$edit and added

if(!$edit){
  $edit = array();
}

becuase edit is initially NULL when uploading a file. If the validation fails (wrong file extension, for example), edit is returned NULL which causes an error in user.module.