Project submission

##### A first stab at project submission (issue #107) that:
+ Links project submission form to home page
+ Adds new projects (and their organizations) as unapproved
+ Keeps unapproved projects (and their organizations) from appearing on site
+ Allows admins to approve projects

*Project submission form*
![image](https://f.cloud.github.com/assets/2766324/1822815/a70bbeaa-716a-11e3-98f3-6b914bc099f2.png)

*ActiveAdmin approval checkbox*
![image](https://f.cloud.github.com/assets/2766324/1823152/f69e8c96-7173-11e3-86dc-60a40d7954ef.png)

*ActiveAdmin approval on show page*
![image](https://f.cloud.github.com/assets/2766324/1823185/f03fcec2-7174-11e3-99d1-890b4547bdf5.png)

*For implementation, it will needs some minimal work in the console to update existing projects:*
```ruby
projects = Project.all
projects.each do |p|
  p.is_approved = true
  p.save!
end
```

##### Potential improvements:
- [ ] Adjust submission form to allow existing organizations to submit new projects
- [ ] Create helper function to parse github urls, allowing for a more intuitive form
- [ ] Allow an organization to submit multiple projects (a + button and add additional form fields?)
- [ ] Create site-wide CSS to highlight flash notices