Add missing initialization methods to MIHBigInteger

Initialization of MIHBigIntegers with hex string and signed ints were missing. Here is a patch with those missing initializers.

    - (id)initWithHexStringValue:(NSString *)hexString;
    - (id)initWithSignedInteger:(BN_LONG)value;

Also another initializer I found to be missing is initializing a new big number with raw bytes ie., `BN_bin2bn`
I saw you have used `BN_mpi2bn` in `initWithData` and that makes sense. But I would like to add an initializer which uses `BN_bin2bn` too, what do you think will be a good name for that initializer?