Make sure entry id can not contain commas

[RGBsoup]

With some pdf-files the generated entry id contains a comma. The extra comma is not allowed in bibtex, which results in a bib file with a syntax error.

This example pdf results in the following invalid bib entry:

@article{fu2005design,,
        title = {Design, fabrication and testing of piezoelectric polymer PVDF microactuators},
        volume = {15},
        issue = {1},
        page = {S141-S146},
        publisher = {IOP Publishing},
        url = {http://dx.doi.org/10.1088/0964-1726/15/1/023},
        doi = {10.1088/0964-1726/15/1/023},
        journal = {Smart Materials and Structures},
        year = {2005},
        month = {12},
        author = {Yao Fu and Erol C Harvey and Muralidhar K Ghantasala and Geoff M Spinks}
}

[MicheleCotrufo]

Thanks for the quick fix! :) I'll merge it right now and release a new version soon.