Uri Template Parser

Addressing issue https://github.com/apiaryio/api-blueprint/issues/78
This needs to be `virtual bool IsExpressionType() const` – same method signature as in the base class `ClassifiedExpression` – otherwise it will result in warning:

```
../src/UriTemplateParser.h:167:14: warning: 
      'snowcrash::LabelExpansionExpression::IsExpressionType' hides overloaded
      virtual function [-Woverloaded-virtual]
        bool IsExpressionType() {
```

See here <https://travis-ci.org/apiaryio/snowcrash/jobs/29191369#L259> 

Note this repeats multiple times bellow...