Add global/local config values for read/write capacity. 
Add global/local config values for read/write capacity. 

Global values is stored in boss.config:
{db_read_capacity, X}, %% X is an integer
{db_write_capacity, Y}, %% Y is an integer


Local values is stored within the model as a function that returns a proplist:

dynamodb_settings() ->
    [
     {range_key, X}, %% X is an atom and should address one of the field within the current model
     {db_write_capacity, Y}, %% Y is an integer
     {db_read_capacity, Z} %% Z is an integer
    ].

QBkira/boss_db,mihawk/boss_db,badubizzle/boss_db