Fix function name conflict

`riakc_datatype` exported a function called `module`. Importing EQC with
`-include_lib("eqc/include/eqc.hrl").` also brings a function called
`module` into the namespace (unqualified). This causes a warning on
R16B02, and an error on R15B01:

R16B02:

`QuickCheck warning: cannot import module/1 because of conflicting
definition.`

R15B01:

`src/riakc_datatype.erl:84: defining imported function module/1`

This simply changes the name of the function `module` to
`module_for_type`. The tests now pass on both R16 and R15.
Merge pull request #157 from basho/bugfix/function-name-conflict

'module' function definition conflict