Improve board functionality

[Cqsi]

I added two methods to the Board class to make it possible to accept and declines challenges even if you aren't a bot. I found this useful because this makes it possible to test whatever you want to create with the Board class against yourself (through creating a challenge against your program). These methods are the exact same ones as those in the Bot class. It should work fine without changing anything.

[codecov-io]

Codecov Report

Merging #20 (ecfbb42) into master (1b000a4) will decrease coverage by 0.32%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##           master      #20      +/-   ##
==========================================
- Coverage   70.04%   69.71%   -0.33%     
==========================================
  Files           8        8              
  Lines         671      677       +6     
==========================================
+ Hits          470      472       +2     
- Misses        201      205       +4     

Impacted Files	Coverage Δ
berserk/clients.py	33.69% <33.33%> (-0.01%)	⬇️

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