Escape % within field's data

[magsilva]

The usage of the symbol "%" which a field should not be a problem for BibTeX data. However, when using Biblatex and Biber, it causes errors. Biblatex and BIber consider % as a comment start symbol. As such, using % within a field, will create malformed BibTeX entries (the parser won't probably find the ending "}," of the field).

An easy solution in to add an escape code "" before each "%".