Table CSV import: fix line breaking issues.

Some Excel version (at least OSX) represent line breaks with only \r instead of
\r\n, which caused import errors.

With this fix the line breaks are normalized beforehand and sniffing csv dialect
as well as URL validation is optimized.
Merge pull request #46 from 4teamwork/jone-tableimport-lineendings

Table CSV import: fix line breaking issues.