polish notation
issue #1 fix: '2a' as a single token
remove ExpressionTree.toString()
fix polish notation
Merge pull request #2 from lekhmanrus/master

expression tree: init
ExpressionTree: rev polish notation for functions
ExpressionTree: more test
Merge pull request #3 from denzp/func-impl

ExpressionTree: rev polish notation for functions
ExpressionTree: binary tree building initial step
ExpressionTree: tests fix
ExpressionTree: more tests
ExpressionTree: reduced tree init
tree api test
ExpressionTree: reduced tree more tests
Merge pull request #5 from denzp/reduced-tree

tree reduce implementation
Update README.md
Update README.md
substraction test