Remove dependency on current platform EOL style.
Remove implements ExpressionInterface
Merge pull request #7 from ezzatron/platform-independant-eols

Remove dependency on current platform's EOL style.
Remove implements ExpressionInterface (fix error)
Merge pull request #8 from startswithaj/patch-1

Remove implements ExpressionInterface
Changed AST to store start/end tokens, and added ParserInterface::parseTokens.

 * Tokens are always stored, allowing access to line/column numbers and
   start/end offsets for both the start and end of the expression that generated
   the AST node.This is more flexible than simply storing the source (fixes #6).

 * Added `ParserInterface::parseTokens()` and changed the lexer to a method-level
   dependency of `parse()` (fixes #9).
Removed unused indentLevel attribute.