Add ACM Digital Libray and IEEE Xplore Digital Library support

[cute-jumper]

I've briefly investigated how to download BibTeX entries from ACM Digital Library and IEEE Xplore Digital Library(I mainly use Google Scholar and these two libraries), and I think it's not hard to implement such a functionality.

Currently I kind of mess up the code for data download and data display. First step is to decouple these two parts, and then I just need to add some code to download data from these two libraries and some code(regular expressions should be quite enough) to find out the titles/subtitles/BibTeX urls.

[cute-jumper]

Implmented in commit 567e94