Improve documentation

[danthorpe]

Finally managed to get Jazzy to run the documentation! This is the command:

$ jazzy --swift-version 2.1 --module Money -x -scheme,Money-iOS

then it works. Currently coverage is 41% - I suspect mostly because of undocumented enums.

[codecov-io]

Current coverage is 100.00%

Merging #27 into development will not affect coverage as of 962cac6

@@            development     #27   diff @@
===========================================
  Files                14      14       
  Stmts              2109    2109       
  Branches              0       0       
  Methods                               
===========================================
  Hit                2109    2109       
  Partial               0       0       
  Missed                0       0       

Review entire Coverage Diff as of 962cac6

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