Support for Taxonomies

[nikolov-tmw]

Support for taxonomies is required.

I've decided to implement the multilingual support for the taxonomies in a similar fashion to how the posts are implemented. This means:

• [x] User can select which taxonomies should be multilingual.
• [x] A duplicate taxonomy is created for each enabled language.
• [x] Relations between terms are stored in the options table, in the form of 1 to many(original term to translation terms) and 1 to 1 relations(translation term to original term), where the key will be something like _mlwp_term_relations_{$term_id}[1 to many] and _mlwp_term_relation_{$term_id}[1 to 1].
• [x] Additional rewrite rules need to be added for the new taxonomies
• [x] The "Fix Posts" page will also need an updater for the taxonomies - the page will be split in two with posts on the left and taxonomies on the right(or tabs can be used as well)

The reason to use separate options, vs one array option is that big chunks of date are not loaded on every page load(when there are hundreds/thousands of categories).

[nikolov-tmw]

A possible problem on the horizon - http://core.trac.wordpress.org/ticket/5809 When initially creating terms with the same name as the current term(for instance when creating the translation terms for the original term) with the same name as the original term, no actual terms are being created - instead that term is simply added to the term_taxonomy relations table.

Workaround for now - append a unique string to the term name - for instance "||bg" where "bg" is the language code for the specific language.

[nikolov-tmw]

Might eventually have to wait until same slugs can be used across different taxonomies, or I'll have to rethink the way additional terms are created right now.