CssLintParser does not map csslint severity

[croensch]

In csslint 0.9.8 two types of violations can be defined, errors and warnings.

csslint --errors=... --warnings=... --format=lint.xml

csslint will report them as strings.

<issue severity="error" />
<issue severity="warning" />

However i dont see that the CssLintParser does map this severity.
I think we should map "error" to "High" and "warning" to "Medium".

[mfriedenhagen]

I will merge #21