Add create_ek() to tpm.rs

[lkatalin]

If this needs more functionality, different return types, etc., please let me know.

[puiterwijk]

This looks decent with just one small comment!

[puiterwijk]

I think the only other value we should consider correct is ecc.
I'd say that the _ => should return an error instead.

[lkatalin]

Here is an updated PR with an error for encryption algorithms other than rsa and ecc, and retrieving the TPM pub object and turning it into a Vec<u8>.

I don't think that turning the TPM pub object into a Vec<u8> should return KeylimeError::InvalidRequest; it seems better to return a TPM(tss_esapi::Error::WrapperError(...WrapperErrorKind::WrongValueFromTPM)), but I wasn't able to create one because it would not let me import WrapperErrorKind. Any thoughts on the best Error type here? If we end up just using an error from Keylime, I will drop the import of the tss_esapi::Error.

[lkatalin]

Depends on #145 #144