Added social media icons functionality to profiles

Summary of changes:
- Added "twitter" and "facebook" fields to "Sign Up" and "My Profile"
pages.
- Added above fields to user model.
- Added social media icons to user page, which will only appear if the
relevant field is completed.
- Added default Facebook and Twitter logos to /app/assets/images.

This is my first attempt at coding in Ruby on Rails, so apologies if
I've missed something obvious, used dodgy/incorrect syntax or if it
doesn't work! I also lack the MySQL knowhow to be able to test it,
unfortunately.
Ignore/disregard this

I added '/.project' to .gitignore so my Aptana config wouldn't be
included in my commits, but I can't work out how to get the GitHub for
Mac client to ignore this change to the .gitignore file!