Initialize Convergence's NSS.js with nss library path (#8314)

glibc's dlopen [treats empty library name string as equivalent to NULL](https://sourceware.org/git/?p=glibc.git;a=blob;f=dlfcn/dlopen.c;h=fa58c4cebff4bc5d2f087b216a6d2c15b1ffa64b;hb=HEAD#l66) and returns an RTLD_DEFAULT handle. OS X' stricter implementation doesn't and fails to load anything when "" is passed as filename. So nss loading via js-ctypes in NSS.js fails on Macs leading  to the console error seen in trac defect 8314.  

Instead of empty string, ssl-observatory can try passing the full library name and if that fails, the full path to libnss3 (in the same way [Convergence initializes NSS.js](https://github.com/moxie0/Convergence/blob/2699d3c4e562983aa500780448ebb3e0bf2384c8/client/components/Convergence.js#L78)