Expression rule implemented; Minor api changes done
Recovery rule improved
Expression rule issues fix
Expression parselets
Calculator example
Calculator simple test implemented; appropriate issues fixed
Calculator-expressions test improved
Expression-recovery test implemented
Rule construction minor improvement
License banners added
Merge branch 'expressions'

- Expression parsing rule based on Pratt's algorithm implemented.
- Several parslet constructors for Expression parser implemented.
- Calculator parser example implemented. As well as appropriate
  functional tests.
- Minor improvements:
  - Avoid rule construction for simple rule nesting cases.
  - Some identifiers were renamed.