Create checkers for contract syntax

[david-yz-liu]

The python_ta.contracts module parses preconditions, postconditions, and representation invariants written as valid Python syntax (see docs). These checks are skipped when the contract is not valid Python syntax. This allows for human language contracts to be written, but can lead to accidental errors if the user intended on writing valid Python syntax.

We should create a new checker, ContractChecker, which checks for valid precondition, postcondition, and RI syntax.