MineTweaker

How to use the Minetweaker integration:
elec332.CustomVillagers.registerVillager(999, "test.png");

elec332.CustomVillagers.addTrade(999, <minecraft:diamond>, <minecraft:clay>, <minecraft:bowl>);

elec332.CustomVillagers.addSpawnData(0, 50, 999);

elec332.CustomVillagers.clearTrades(1);

elec332.CustomVillagers.addTrade(1, <minecraft:emerald>, <minecraft:bowl>);

The first line registers an villager with villager ID 999, and the image named: test.png
The second line adds a trade to villager ID 999, the villager trades a diamond and a clay block for a bowl.
The third line adds spawn data for villager ID 999 (The one we just created). What this line basically does, is: when a villager with villager ID 0 (The "standard" villager) spawns, it has a (specified in the 2nd number) 50% chance of spawning a villager with villager ID 999 (our villager) instead. this is unfortunately the only way of making villagers spawn, but it also has its advantages -You can set it to 100% to completely replace a certain villager.-

The forth line removes/clear all trades from villager ID 1 (I think the blacksmith, but dont quote me on that ;))
And the last line add a trade to villager ID 1; an emerald for a bowl. (Meaning that Villager ID 1 / the blacksmith will only have this trade).

NOTE: the clearTrades is very buggy/might not work (as intended) at the moment, this is being worked on!