Allow users to set target URL

[voidshard]

• Added SetAPIPath function - seems helpful to allow users to set target URL if they're running their own PokeAPI server
• Added error handling around 4xx and 5xx codes (including HTTP retries, backoff etc), pretty sure this addresses https://github.com/mtslzr/pokeapi-go/issues/29 as we now raise when a 4xx code arrives rather than trying to parse no data
• Fixed two failing tests -- suspect due to changes on remote end (should probably mock API responses as part of the tests)

[codecov-io]

Codecov Report

Merging #30 (43226df) into master (ee7a30b) will decrease coverage by 6.60%. The diff coverage is 59.45%.

@@            Coverage Diff             @@
##           master      #30      +/-   ##
==========================================
- Coverage   97.05%   90.44%   -6.61%     
==========================================
  Files          14       14              
  Lines         204      178      -26     
==========================================
- Hits          198      161      -37     
- Misses          3       11       +8     
- Partials        3        6       +3     

Impacted Files	Coverage Δ
client.go	61.36% <59.45%> (-7.06%)	:arrow_down:
cache.go	100.00% <0.00%> (ø)
games.go	100.00% <0.00%> (ø)
items.go	100.00% <0.00%> (ø)
moves.go	100.00% <0.00%> (ø)
berries.go	100.00% <0.00%> (ø)
pokemon.go	100.00% <0.00%> (ø)
utility.go	100.00% <0.00%> (ø)
contests.go	100.00% <0.00%> (ø)
machines.go	100.00% <0.00%> (ø)
... and 4 more

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