Use table_prefix_name for plugins' table_name

This makes Noosfero::Plugin::ActiveRecord obsolete. With plugin
interdependency, self.inherited may get the table name from another
class.
Merge branch 'use_table_prefix_for_plugins' into 'master'

Use table prefix for plugins' table_name

See full commit message

See merge request !322
Support running Noosfero as a /subdirectory

- Remove absolute URL's from stylesheets and javascripts
- Handle prefix in LinkListBlock
- add Noosfero.root method as a central data source

The application server needs to be started with the
RAILS_RELATIVE_URL_ROOT environment variable set to the desired prefix.

In development, this can be tested with

  # assuming /social prefix
  $ RAILS_RELATIVE_URL_ROOT=/social thin --prefix /social start

For production, see INSTALL.md