Allow custom periods in seconds in timeInHex

[amdelamar]

Instead of forcing 30s default periods, this
change allows users to specify their own
period value using timeInHex(time, period).

Period must be positive non-zero integer.

Fixes #14

[codecov]

Codecov Report

Merging #24 into master will decrease coverage by 0.59%.
The diff coverage is 80.00%.

@@             Coverage Diff              @@
##             master      #24      +/-   ##
============================================
- Coverage     94.48%   93.89%   -0.60%     
- Complexity       46       47       +1     
============================================
  Files             5        5              
  Lines           127      131       +4     
  Branches         21       22       +1     
============================================
+ Hits            120      123       +3     
  Misses            7        7              
- Partials          0        1       +1     

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/com/amdelamar/jotp/OTP.java	88.57% <80.00%> (-0.83%)	21.00 <2.00> (+1.00)	⬇️

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