Fix arxiv url extraction

[mttk]

Fixes #23

Arxiv changed their layout so I updated the line indices / starting points accordingly. Gave them names so further updates might be easier to update & debug. I tested briefly on Ubuntu / python 3.6, but this is raw string indexing so it shouldn't break anything cross platform.

> sotawhat object detection
Improve Object Detection by Data Enhancement based on Generative Adversarial Nets (Wei Jiang - 5 March, 2019)
The accuracy of the object detection model depends on whether the anchor boxes effectively trained. Our model with 321x321 input achieves 78.7% mAP on the VOC2007 test, 76.6% mAP on the VOC2012 test.
Link: https://arxiv.org/abs/1903.01716

[chenquan]

@chiphuyen Request merging