Updates amazon.ion dependency to >=0.5, adds ion-hash-test submodule, and minor cleanup

[pbcornell]

Continuation of effort started in https://github.com/amzn/ion-hash-python/commit/d6742fa4b1b9f0cec0e4cd3f07434396dd09e23c. Resolves https://github.com/amzn/ion-hash-python/issues/10.

• bumps amazon.ion dependency to >=0.5
• passes additional, required parameter to _dump
• adds ion-hash-test submodule
  • updates tests to reference test files from the submodule instead of local copies
  • removes local, duplicate copy of ion_hash_tests.ion
  • removes local, duplicate copy of big-list-of-naughty-strings.txt
• removes unused methods from test_big_list_of_naughty_strings.py
• adds egg directory to ignore

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