Refactor message module to better reflect other crypto libraries

[roks0n]

Proposed changes

The following change resolves #30 and #35, where verify() method in the message raises an exception again and refactors the message module to reflect other crypto libraries.

After completing these changes I've run the TransferBuilder and successfully made a transaction - this is just to prove that nothing regarding signing didn't break.

Types of changes

What types of changes does your code introduce?

• Bugfix (non-breaking change which fixes an issue)
• Refactoring (improve a current implementation without adding a new feature or fixing a bug)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Test (adding missing tests or fixing existing tests)

Checklist

• I have read the CONTRIBUTING documentation
• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

[codecov-io]

Codecov Report

Merging #36 into develop will increase coverage by 0.24%.
The diff coverage is 86.95%.

@@             Coverage Diff             @@
##           develop      #36      +/-   ##
===========================================
+ Coverage    77.51%   77.76%   +0.24%     
===========================================
  Files           38       38              
  Lines          676      697      +21     
  Branches        60       62       +2     
===========================================
+ Hits           524      542      +18     
- Misses         126      128       +2     
- Partials        26       27       +1

Impacted Files	Coverage Δ
crypto/exceptions.py	100% <100%> (ø)	⬆️
crypto/message.py	100% <100%> (ø)	⬆️
crypto/identity/private_key.py	100% <100%> (+11.76%)	⬆️
crypto/transactions/builder/base.py	92.3% <100%> (ø)	⬆️
crypto/resources/transaction.py	85% <45.45%> (-3.5%)	⬇️

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