Changed the name of the timeout macro in riakc.hrl from DEFAULT_TIMEOUT to DEFAULT_PB_TIMEOUT.

The name DEFAULT_TIMEOUT is confusing, because riakc.hrl is used in both the riak-erlang-client and riak-erlang-http-client packages. Adding PB to the macro name makes it clear to the user they are changing the default timeout for the protobuf client and not http.
Merge pull request #140 from NineFX/rename-default-timeout-macro

Changed the name of the timeout macro in riakc.hrl