Adding some utilty functions like `t.sha3`, `t.u256_to_string`, `t.string_to_u256`

They have some tests too. The name of the latter two could be better. Helper functions fill some holes in [this wiki page](https://github.com/ethereum/pyethereum/wiki/Using-pyethereum.tester) describing the functions interesting for testing. (#159 for reference)

Btw, `x-2**256 if x > 2**255 else x` in "`s.send`" could be like something that bites people as unexpected? (hmm if it stays i should mention it in the wiki page)