added memo with tests and verify_signature bool

[crypt0miester]

• Added memo program with tests.
• Added memo docs
• Added verify signatures for transaction.serialize() since sometimes the frontend is the fee payer.
  This uses the same functionality as web3.js
• Added unit test to make sure that the verify signature=False would work.

[codecov]

Codecov Report

Merging #249 (1e1c413) into master (8969968) will increase coverage by 0.08%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #249      +/-   ##
==========================================
+ Coverage   91.02%   91.11%   +0.08%     
==========================================
  Files          34       36       +2     
  Lines        2541     2565      +24     
==========================================
+ Hits         2313     2337      +24     
  Misses        228      228              

[michaelhly]

@kevinheavey everything looks fine to you?

[kevinheavey]

Yeah lgtm