Stable release

Refine the syntax of separation logic:

• VariableExpression is deprecated, Variable is used instead of VaraibleExpression.
• EqNullTerm, NEqNullTerm, EqTerm, NEqTerm are deprecated. They will be replaced by ComparisonTerm.
• Add NullExpression
• Optimizing the alias container using HashMap instead of LinkList<LinkedList>. Hopefully it will be faster.
• Change also the antlr grammar .g4 file.
• All unit tests are OK.

Stable release

Work with jpf-star v0.1b and v0.2