Come up with better consistent names for getStart/getEndPosition

[roblourens]

https://github.com/Microsoft/tolerant-php-parser/pull/156#discussion_r122614440

[TysonAndre]

In the meantime, could you add an alias to either of the classes? (e.g. add getStartPosition() to Node, keeping the existing getStart() method and maybe marking it as deprecated).

Adding a new alias should be backwards compatible, and lets users get an offset without doing an instanceof check

[TysonAndre]

The Node and Token methods were changed in #349