Add Cangjie 3 code "raiu" for character "嘅"

This is a forward-port of Koala's change:

    commit 8f1c9ad57579dbf3618fb5beaaade7ce91b815b7
    Author: Koala Yeung <koalay@gmail.com>
    Date:   Thu Nov 20 19:03:42 2014 +0800

        Add Cangjie 3 code "raiu" for character "嘅"

        The character "嘅" has a different glyph in different Chinese
        standard. The mapping to `口日戈山` ("raiu") makes better sense
        for the Hong Kong glyph standard.

        The mapping exists in Cangjie 5 already. It is now added to Cangjie
        3 also.

        Fixes #93.
Add INSTALL and README files
build: Fix distcheck

I got these wrong when I added data/cangjie.db
build: Depend on Gom >= 0.2.2

We need such a recent version for the support of write transactions.