Merge Fork of WordPress plugin with significant user interface and performance enhancements

Went to use the WordPress plugin for my own site, and ran into several compatibility issues. Forked the SVN repo from WordPress.org, and am now hoping to merge my changes back up stream. Discussed briefly with Beaver6813 via e-mail, but am submitting a pull request in case this is easier to merge.

Fork should be fully backwards compatible with previous versions.

Full change log and screenshots: https://github.com/benbalter/Infinite-Scroll/tree/develop

**Key Changes:**
* Added support for internationalization (see FAQ for information on translating)
* Added support for custom post types (now works on all pages but is_singular())
* Options screens significantly simplified with additional help text
* Presets screen now uses standard WordPress administrative interface
* Ability to submit your theme's CSS Selectors to the community CSS selector database to aid others with installation
* Changes to presets are now made inline; saved via AJAX without page reload
* Added additional API endpoints for developers to modify and customize plugin behavior
* Loading image now uses native WordPress uploader and can accept arbitrary URL or image from media gallery
* Loading and finished text now uses native WordPress TinyMCE editor
* If site administrator has not entered CSS selectors and preset is known, plugin will default to preset
* Any preset entered by site (or network) administrator will now override community contributed presets by default
* Community contributed CSS selector presets are now stored in the database allowing for plugin directory to be unwritable (security enhancement)
* Prompts users to default to CSS selector preset when available
* Presets now support child themes
* Community contributed CSS selected asynchronously update daily (performance enhancement)
* Site (or network) specific CSS selector presets are now stored in the database (security enhancement)
* CSS Preset updater now uses WP_HTTP class (compatibility enhancement)
* Javascript file now served directly to user (rather than proxied via PHP) to allow for browser caching, minification, serving via CDN, and better integration with caching plugins (performance enhancement)
* Javascript options stored natively in database and passed directly to script via WordPress's wp_localize_script function (performance and customizability enhancement)
* No longer relies on deprecated user levels to determine permissions
* Default loading and fished messages changed
* Fixes for error and warnings when run with WP_DEBUG enabled
* Removed prompt for option to activate infinite scrolling for only certain users (defaults to all)
* Removed prompt for option to toggle debug mode (defaults to WP_DEBUG or SCRIPT_DEBUG)
* Removed prompt for option to toggle scrolling behavior (defaults to automatic)
* Removed prompt for additional callbacks
* Removed prompt to select image alignment (defaults to left, can override via CSS)
* Significant code clean up, file reorganization, and in-line documentation to conform to WordPress coding and style standards (whitespace, double v. single quotes, tab drift, trailing commas, closing PHP tags, etc.)
* Classes loaded as sub-classes (rather than extending parent class) to prevent classes from becoming out of sync
* Added phpdoc style documentation
* Plugin file name changed to conform to standard WordPress naming conventions