Tests and minor improvements to ciphers module

[sarathsoma]

Short description of what this resolves:

Minor tweaks to how yoda exits during invalid inputs for ciphers module.
adds tests for ciphers module.

Changes proposed in this pull request:

• Exit with status != 0 during invalid inputs
• Tests to ciphers module [ ceaser, atbash and rot13 ]

Status (please check what you already did):

• made PR ready for code review
• added some tests for the functionality

[sarathsoma]

@manparvesh Does this look good ? Can i work on adding more ciphers and tests ?

[manparvesh]

@sarathsoma the tests are failing. Please fix that before adding more ciphers

[sarathsoma]

@manparvesh Ran the tests on the dev environment. Everything looks okay. No tests failed. Any leads on this ?

[manparvesh]

@sarathsoma I reran the build and found that it was okay for python 2 but not for 3. For Python 3 it probably went into some infinite loop. Please take a look:
https://travis-ci.org/yoda-pa/yoda/builds/458736952

I am guessing you have python 2 in your machine, please try on python 3 as well

[codecov-io]

Codecov Report

Merging #235 into master will increase coverage by 2.82%.
The diff coverage is 93.18%.

@@            Coverage Diff             @@
##           master     #235      +/-   ##
==========================================
+ Coverage   76.78%   79.61%   +2.82%     
==========================================
  Files          60       64       +4     
  Lines        3442     3522      +80     
==========================================
+ Hits         2643     2804     +161     
+ Misses        799      718      -81

Impacted Files	Coverage Δ
modules/ciphers/atbash.py	100% <100%> (+70%)	⬆️
modules/ciphers/caesar.py	95% <100%> (+74.48%)	⬆️
modules/ciphers/rot13.py	94.28% <100%> (+70.75%)	⬆️
modules/dev.py	88.05% <50%> (+4.01%)	⬆️
tests/ciphers/test_rot13.py	94.44% <94.44%> (ø)
tests/ciphers/test_atbash.py	94.44% <94.44%> (ø)
tests/ciphers/test_input.py	94.44% <94.44%> (ø)
tests/ciphers/test_ceaser.py	95.45% <95.45%> (ø)
tests/dev/test_horoscope.py	92% <0%> (-4%)	⬇️
modules/gif.py	70% <0%> (-3.34%)	⬇️
... and 11 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update f9e5b6b...8d17f4f. Read the comment docs.