Refactor the package structure and add document

[codecov[bot]]

Codecov Report

Merging #20 (99913f9) into develop (2e13db2) will increase coverage by 0.25%. The diff coverage is 40.00%.

@@             Coverage Diff             @@
##           develop      #20      +/-   ##
===========================================
+ Coverage    41.21%   41.47%   +0.25%     
===========================================
  Files           13       13              
  Lines          296      299       +3     
===========================================
+ Hits           122      124       +2     
- Misses         165      166       +1     
  Partials         9        9              

Impacted Files	Coverage Δ
client/pixeldrain_api_client.go	76.31% <ø> (ø)
cmd/pd/command/download.go	0.00% <ø> (ø)
cmd/pd/command/list.go	0.00% <0.00%> (ø)
cmd/pd/command/upload.go	0.00% <ø> (ø)
create_list.go	88.00% <ø> (ø)
download.go	58.62% <ø> (ø)
error.go	78.57% <ø> (ø)
models/standard_error.go	0.00% <ø> (ø)
transport.go	54.16% <ø> (ø)
upload.go	71.42% <ø> (ø)
... and 1 more