Add composer suggestion for bcmath, gmp extensions or ...

[mcd-php]

... make the project strictly fail installing by separation into three packages:

• base58php requires base58php-service
• base58php-bcmath provides base58php-service
• base58php-gmp provides base58php-service

( see provide )

[stephen-hill]

Hi

I've given this some thought and I think I'll add the "suggest" property.

While creating additional packages might be the correct route to go down, I think it adds extra complexity to such a simple library.

Cheers
Stephen