Fix 3D box orientation post-processing

Use arctan2 instead of arctan.

Fix typo

FIx typo.

Add link to a keras implementation.

ctdet_decode support cat_spec_wh

fixed some tiny bugs

Fix typo

pascal voc testing requirements

These packages are needed when working with pascal voc

make the function signature consistent

Add comments to explain why nms = None

Explicit is better than implicit.

Revert "from model.nms_wrapper import nms for lines 132 and 168"

from model.nms_wrapper import nms for lines 132 and 168

[flake8](http://flake8.pycqa.org) testing of https://github.com/xingyizhou/CenterNet on Python 3.7.1

$ __flake8 . --count --select=E9,F63,F72,F82 --show-source --statistics__
```
./src/lib/models/networks/dlav0.py:417:5: F821 undefined name 'dla'
    dla.BatchNorm = bn
    ^
./src/tools/voc_eval_lib/model/test.py:132:14: F821 undefined name 'nms'
      keep = nms(dets, thresh)
             ^
./src/tools/voc_eval_lib/model/test.py:168:14: F821 undefined name 'nms'
      keep = nms(cls_dets, cfg.TEST.NMS)
             ^
3     F821 undefined name 'dla'
3
```
__E901,E999,F821,F822,F823__ are the "_showstopper_" [flake8](http://flake8.pycqa.org) issues that can halt the runtime with a SyntaxError, NameError, etc. These 5 are different from most other flake8 issues which are merely "style violations" -- useful for readability but they do not effect runtime safety.
* F821: undefined name `name`
* F822: undefined name `name` in `__all__`
* F823: local variable name referenced before assignment
* E901: SyntaxError or IndentationError
* E999: SyntaxError -- failed to compile a file into an Abstract Syntax Tree

Add automated testing on Python 3.6 and 3.7 on Travis CI

The owner of the this repo would need to go to https://travis-ci.com/profile, log in with GitHub credentials, select the __free plan__, and flip the repository switch on to enable free automated testing of all pull requests.  Travis will lint with flake8 and then run __python runtests__.

Update README.md

tackle word misspelling issue