Add ability to read pkcs#12 certificate files

[gothub]

This capability is needed for Mac OS X Yosemite (10.10.x) which has an Apple
supplied version of libcurl (7.37.1) that no longer supports PEM format files. For more
information, see http://curl.haxx.se/mail/archive-2014-10/0053.html

[jeroen]

@gothub I have implemented this in the openssl package. LMK if you have any feedback:

install.packages("https://github.com/jeroenooms/openssl/archive/master.tar.gz", repos = NULL)

Try ?read_pkcs12 for the man page.