Update API doc types for Bloom filter methods

[chikeichan]

Fixes #332

Using the types that are expected in utils.keccak256, which is what add and check uses to process to params

[s1na]

There is a discussion (ethereumjs/ethereumjs-util#172) on making the API stricter by allowing only key types as parameters. Keccak256 wasn't mentioned and I assume the plan is to have it accept various parameter types, in which case this fix is justified. If it's decided to go with a stricter keccak256, we'd have to re-visit the docs here and fix them, but I think that'll take some time. We can probably merge this for now. What do you think @holgerd77?

[holgerd77]

Thanks Jacky! @s1na yes, think so too, so looks good for now.