Added BIP62 rule #6 support and test case derived from existing test transactions.
Implemented BIP62 rules 2, 3 and 4 based on Toshi code for malleability checks on transactions.
Added DER signature format validation and unit tests.
Added check to verify that the S-value on a DER signature is low.
Added test cases for compare_big_endian().
Enabled public key and signature BIP62 format checks when validating signatures.
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fix typo in README. close #144
minor cleanup in Script.compare_big_endian