Converting variables to static

Also, indenting class variables only with spaces as proposed by @greguly
in b2af3c0277d6f110ecbf467d12283253cce0e76d
Flushing URLs on activation and deactivation
Calling add_meta_box at the right time

Undoing ac0d7f2747f8ac05822d43a94a4365cdd1498408, as the meta box class
needs to schedule the `save_post` action at the right time. That would
be on `admin_init`.

Calling `add_meta_boxes` directly won't trigger `save_post` of
`Glossario_Metabox`, and so the plugin fields won't be saved.
Render terms table using jquery-data-tables
Merge branch 'master' of github.com:wpbrasil/glossario