Move class file into includes subdirectory

This is a standard directory location for included files.
Remove email address

Name and URLs are enough to be able to make contact. All issues should be reported to the right place anyway,
and that isn't email.
Change `@package` tag to WordPress coding standards format

Underscore, not camel case.
Switch away from Singleton approach

Moves add_*() calls out of constructor.
Simplify loading of plugin text domain

The `load_plugin_textdomain()` is now a single line, so it doesn't need
to be in a wrapping function or method. It is moved to the
`plugins_loaded` hook, which is considerably earlier than init, so any strings
defined before then (even if not output) will be translatable too.

The removal of the explicit `load_textdomain()` and variables comes from
https://core.trac.wordpress.org/changeset/25059 which automatically
loads (for example)
wp-content/languages/plugins/genesis-header-nav-en_GB.mo instead of
wp-content/languages/genesis-header-nav/genesis-header-nav-en_GB.mo .
This is therefore a backwards compatibility change but one which
supports the upcoming language packs. It bumps the WP requirement
to 3.7. Move any existing custom .mo to the new location for it to work.

The change from `basename( dirname( __FILE__ ) )` back to the more
standard `dirname( plugin_basename( __FILE__ ) )` comes from the fixes
in https://core.trac.wordpress.org/ticket/16953 which make
`plugin_basename()` resolve symlinked plugins. This bumps the requirement
for WP to 3.9.
Split up initial functions

Only need to load the text domain early. Leave everything else until a later, to give
themes a chance to apply priority filter.

Fixes #3
Update README
Add documentation, amend initial function hook, simplify nav menu registration
Use namespaces, because 2014

Bumps requirements from WP default of PHP 5.2.4 to 5.3.
Avoid using any global variables
Merge branch 'feature/rewrite' into develop