Major changes

Adds serialization support through the following methods (#63):
- to_h: Serialize into a Ruby hash (with symbol keys). Accepts a boolean argument, representing whether or not to deep-serialize nested Author and Category objects (defaults to false).
- as_json: Serialize into a Ruby hash which is also a valid JSON hash.
- to_json: Serialize into a valid JSON string.
Remove version filter from query ID list. (#69)
Previously, all of the following would return the latest paper, 1807.06918v2:
```
Arx.get('1807.06918').version #=> 2
Arx.get('1807.06918v1').version #=> 2
Arx.get('1807.06918v2').version #=> 2
```
Adds ATTRIBUTES constant for Paper, Author and Category entities, as a list of which attributes are available for the entity. (#63)
Remove key-word arguments from Paper#id and Paper#url. (#70)
Previously, Paper#id and Paper#url accepted a version key-word argument, which was a boolean variable indicating whether or not to include the version number in the ID or URL.

This has now been changed to a regular argument, which still defaults to false.
```
paper = Arx.get('cond-mat/9609089')

# Old (no longer works)
paper.id(version: true)
paper.url(version: true)

# New
paper.id(true) #=> "cond-mat/9609089v1"
paper.url(true) #=> "http://arxiv.org/abs/cond-mat/9609089v1"
```
Add equality operator (==) to entities. (#68)

Minor changes

Add more category mappings to CATEGORIES. (#71)
Add licensing information to README.md under the Acknowledgements section. (#66)
Add yard development dependency for documentation. (#65)
Make documentation spacing uniform. (#64)
Coveralls:
- coveralls [= 0.8.22 to = 0.8.23] (#62)
- thor [~> 0.19.4 to ~> 0.20.3] (#67)