bugfix - Changing from singleton model to returning query object (which is lazy eval'ed)

### Background
1. This pull request is to follow up on https://github.com/NathanielWroblewski/helloblock/issues/4
2. Some preliminary thoughts were shared here: https://gist.github.com/sidazhang/cb1ee9a2526b3eb382a9

### Details
The design of the library rests on a singleton module where there is one module level variable that keeps track of the state. This combined with lazy eval means that it returns itself (as a class) into a variable. 

However, because the query is only fired upon ```to_hash``` or ```[]```, the stateful nature of lazy eval combined with a module level variable which must be cleared upon every call means that this doesn't work very nicely. Everytime you call the variable, it will make another api call (except without the params since its already cleared)

```ruby    
response = HelloBlock::Transaction.find(tx)
p response.class == HelloBlock::Transaction
# since response is the class itself, everytime you use the variable it will make another call
```  

### Proposed solution
This is a proof of concept pull request which turns ```module Query``` into a ```class Query``` and have resource classes (such as Transaction, Address etc) inherit from it. Each call returns an instance which also keeps track of state such as what is the query params, and whether the call has been made and cached.

### Tests
All the integration tests passed (so we know this works) although some of the unit tests are still failing. Since this is a structural change to the library I don't expect many of the unit tests to still be relevant. 
```bash
➜  helloblock git:(bugfix-singleton) rspec spec/integration/api_calls_work_spec.rb
You are using WebMock 1.16.1. VCR 2.8.0 has been tested against WebMock >= 1.8.0, < 1.16, and you are using a newer version. If you experience VCR issues, consider downgrading WebMock as it may fix it.
..................

Finished in 0.159 seconds
18 examples, 0 failures
```