Tests for hmac
shadowing of hmac function fixed.
add IsString instance for HMACKey.
hmac and hmac' take HMACKey as input

- now that there is IsString instance for HMACKey this should be fine
  for the user
fixed hmac tests.
Merge added hmac and hmac' functions.
x-raaz-core now added to the travis build scripts
Added gadget and primitive composition
Benchmarks for Sha