Parse Eval: re-implement parser
Parse Eval: add math_level to eval_operators
Parse Eval: accept eval statements starting with a string
Parse Eval: fix operator regexp; add more escape characters
Parse Eval, String, Condition: Change ret.text_length to ret.content
Bugfix: parsing property values with unit

In 3b4a009a7c019adb31a19d21380f20203f674c91 function 'number' was renamed to 'metric'
Parse String, Eval, Condition: change return type to out parameters: result and content
Eval: re-implement ==, !=, ne, eq
Parse Selectors: read MapCSS 0.2 compatible link selectors
Parse/Compile Selectors: copy conditions on role/sequence_id to link selector
Parse link selector: catch exception if case of empty link selector -> avoid error message
Support way -> node link selectors
@values: use , as separator
Ignore all unknown @-statements