I have just found this same extension installed and I can confirm that I didnt install it (atleast knowingly) and working in IT I'm fairly aware of what I install or the system prompts me for. The only reason I came across this is that it appears it may have been causing safari to crash when I right click on a link and select open in new tab. Haven uninstalled it and safari is a happy camper again
It may have been installed when you either installed or updated Vuze, the popular Mac bittorrent client. It has an install option in the install wizard which looks very like a normal white page T&C text but is not for Vuze but for 'Free extensions by Spigot' including Searchme extension.
In that text it does indicate all the extensions added, default search engine changed etc. Also it adds Amazon and eBay buttons to the toolbar (presumably with referral codes for click throughs)
Just remember it goes through this wizard for every subsequent update also of Vuze.
If installed you will need to manually reset the default search engine and homepage in Safari preferences back to your original choices. Then remove the three extensions via Safari > Preferences > Extensions. Or manually trash them from ~/Library/Safari/Extensions/.
Or it could have been installed by a similar adware type install of another app. Either way check carefully all your Safari settings.
I also suffered some real problems with trying to right click select text in Safari and suspect this was the cause as the issue has gone since removing it.