More generic layout for caching.

- new $wgBugzillaCacheType, replaces $wgBugzillaUseCache and $wgCacheObject
  with mysql, apc, memcache, postgresql, sqlite, dummy types;
- new Bugzilla::getCacheClass() and ::getCache() static methods;
- new BugzillaCacheI::setup() abstract method, called by BugzillaCreateCache();
- new BugzillaCacheSql class, handling any generic MW SQL-backend;
  BugzillaCacheMysql is dropped;
- separate SQL CREATE TABLE definitions for MySQL, PostgreSQL and SQLite;
- base64_encode/decode cached values, fixes SQL escaping issues;
- drop cache db fetched_at column.
Merge pull request #13 from rdalverny/rework_cache

More generic layout for caching.