Transform into Ruby client library for AvaTax

This pull request is designed to bring this repository to a point where it can exist as a proper Ruby client library for access the AvaTax REST API. All the pre-work to make this publishable as a gem has been done, **except** adding a Avalara-friendly description for the gem and setting a homepage.

This pull request makes it possible for a Ruby developer to
```shell
$ gem install avatax
```
```ruby
require 'avatax'
tax_service = AvaTax::TaxService.new
... # build my document
result = tax_service.get(doc)
```

This will, by default, attempt to pull Account Number and License Key information from environment variables, which is the preferred configuration method for deploying to services like Heroku (as I plan to do). Alternatively, a Ruby developer may specify a file containing the credentials:
```ruby
require 'avatax'
AvaTax.configure_from 'path/to/my/credentials.yaml'
tax_service = AvaTax::TaxService.new
... # build my document
result = tax_service.get(doc)
```