added meta_description field to fiber Page model, added migration

fixed some weird migration bug on Page model 'is_public' field, migration 0010 adds 'is_public' field, migration 0011 had none 'is_public' field
added Page 'meta_description' field to django admin
added meta_description to Fiber front-end admin
Merge pull request #148 from nvandijk/feature/page-meta-description

Page meta description