Add code for 'Challenge POS'

Install gems using `bundle install`.

Tests can be run using `guard`. Use the command `all` to run all tests, or save
a specific file in lib/ or test/ to run a single test case.

To run the app, first create the database then run `puma` to start the server:
```shell
rake create_table
puma
```
Navigate to http://localhost:9292 in your browser.