Add documentation and tests

[pacoorozco]

Added

• A new Logger has been added to be shared across the whole package. See logger.go.
• Tests for almost all the code, except uploads.go that needs a lot of refactor.
• Package's documentation and examples of use.

Changed

• xerrors is used instead of errors. Preparing code for Go 1.13 release.

Deprecated

• Token() method has been deprecated. Current package implementation doesn't need to have OAuth token.

[codecov-io]

Codecov Report

Merging #21 into master will increase coverage by 9.04%. The diff coverage is 34.04%.

@@            Coverage Diff             @@
##           master      #21      +/-   ##
==========================================
+ Coverage   11.32%   20.37%   +9.04%     
==========================================
  Files           5        6       +1     
  Lines         265      270       +5     
==========================================
+ Hits           30       55      +25     
+ Misses        234      214      -20     
  Partials        1        1

Impacted Files	Coverage Δ
lib-gphotos/albums.go	0% <ø> (ø)	:arrow_up:
lib-gphotos/uploads.go	0% <0%> (ø)	:arrow_up:
lib-gphotos/progress.go	93.75% <100%> (ø)	:arrow_up:
lib-gphotos/logger.go	100% <100%> (ø)
lib-gphotos/client.go	100% <100%> (+100%)	:arrow_up:
lib-gphotos/oauth.go	100% <0%> (+100%)	:arrow_up:

---

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 03c8bfb...b69335d. Read the comment docs.