Switching to RSpec for testing

This is the first installment of the potential switch, it illustrates the format of model testing. If anyone has any input on how I can make these tests even better, while maintaining readability, I would really appreciate the help. I purposely only tested one method so that we can focus more on the style versus what is actually being tested. I'm hoping to decide on a style before this PR is merged.  @ferrous26 @zhangsu
By tailoring the description, do you mean something like:
```ruby
it 'repo_name should start with "group_"' do
   ...
end
```
This will look fine in the formatted output, but it doesn't read naturally in the code ('it repo_name' should really be written as 'its repo_name' in English). It looks better if you have this instead:
```ruby
example 'repo_name should start with "group_"' do
   ...
end
```
`example` is just an alias of `it` but sometimes make the code more readable. It should be in `rspec-core`.
The use of `its` is my preference. I'm okay if you insist on using only the core RSpec gems. After all, the RSpec core gems themselves don't use external gems for their own specs.