So Sauron was the Necromancer?

[Eorl of Rohan]

I read The Hobbit and Silmarillion when I was little, and somehow never had the chance to re-read it even when I read and fell in love with LOTR. I've just re-read them for the first time ever, and it's given me a few surprising questions that everyone except for a newbie like me probably knows the answer already:

In the Silmarillion, there is a mention of the wisest of the Maiar being Olorin. If my memory does not fail me (as well it might) isn't that the name that Gandalf Mithrandir Stormcrow is called by in the... was it East? And to the South he goes not? Anyway, is this just a coincidence of names, or is Gandalf the manifestation of the Maia Olorin?!

In the Hobbit, Gandalf mentions something about Bilbo making sure not to approach the realm of the Necromancer. Back when I first read it, I kept wondering who this Necromancer was and why he never comes into the story. (I mean, in fairy tales, if someone tells a princess to steer clear of an evil witch, that evil witch /always/ appears. But Gandalf just warned about this necromancer guy, and he never makes an appearance.) But then it just hit me... Could this Necromancer be an oblique reference to Sauron?!