Add model generator

[satoryu]

This pull request provides model generator.

bundle exec rails g model post title:string created_at:time -o redis_orm

this generator creates model file. in the above case, the following model is created.

class Post < RedisOrm::Base
  property :title, String
  property :created_at, Time
end

[german]

@satoryu Thanks for your code, excellent work! I like the way you organized spec folder and I'll move all tests into it eventually. The only thing I corrected is the version of rails in .gemspec file.