Add spec errata
Merge pull request #60 from dionyziz/errata

Add spec errata.
Parse test: Refine precedence tests for simple epxressions.
Parse test: Refine precedence tests for signs.
Parse test: Refine precedence tests for pow.
Parse test: Refine precedence tests for multiplicative operators.
Parse test: Refine associativity tests for additive operators.