Allow scientific notation in 'real' literals

More details on a task 2

Start documenting semantic analysis and compilation of integer literals

Don't print the syntax tree with toString

Merge pull request #18 from romildo/develop

Develop

Add converstions toTree in ASTs

Overload toString method in AST class using commons-lang3 library

Let the parser construct abstract syntax trees

Add semantic actions to the grammar

The terminal symbol for literal number and the non terminal symbols receives a semantic value of type Double, corresponding to their value when evaluated. The semantic actions evaluate the operations indicated in each production rule, calculating the semantic value of the symbol in its left hand side. At the end the start symbol semantic value is the value of the whole expression.

Override error routines in parser