Modified Spinner values for CreateKeyDialog. Added info for user about custom key length restrictions.

This is a reference to additional feature mentioned on pull request https://github.com/open-keychain/open-keychain/pull/536. 

I've added custom `Spinner` values for every algorithm. RSA and DSA have value "custom", that displays `EditText` below. There is a small bug here: when user has selected RSA/DSA and custom key length and he decides to switch to ElGamal, custom key length EditText disappears (as expected), but keyboard remains on screen). That does not affect functionality, but it's just "not cool" for user experience. 
I've tried to hide keyboard before, on my own project, and I had a lot of trouble with it. Is there any smart way for doing that?