remove redundant http:// prefix from addresses

riak_init() already supplies the protocol portion of the address
making its use in the other functions redundant and incorrect.
add missing install line for riackerrors.h header
use dynamically allocated buffers

replace the use of statically-sized, stack based variables
with dynamically allocated buffers.
the curl writefunc will resize the payload buffer if needed.
mark char* arguments as const where appropriate

multiple functions accept a user-supplied char* as an argument.
The library functions do not (and should not) modify these arguments
so we mark the parameters as const to reflect that.