Make paging columns suck less

Adds a new utility method in cassandra.js:

```javascript
/**
 * Query a page of data from the columns of a specified row.
 *
 * @param  {String}     columnFamily        The column family to query
 * @param  {String}     keyColumnName       The name of the key column to query
 * @param  {String}     keyColumnValue      The value of the key column to query
 * @param  {String}     [start]             The *exclusive* starting point of the query. If unspecified, will start from the beginning.
 * @param  {Number}     [limit]             The maximum number of columns to fetch. Defaults to 25
 * @param  {Object}     [opts]              Advanced query options
 * @param  {String}     [opts.consistency]  The Cassandra consistency with which to query. Defaults to QUORUM
 * @param  {Boolean}    [opts.reversed]     Whether or not the columns should be queried in reverse direction (highest to lowest). If `true`, the `start` range should be the *high range* from which you wish to start return columns. Defaults to `false`
 * @param  {String}     [opts.end]          The *inclusive* ending point of the query. If unspecified, will query columns to the end of the row
 * @param  {Function}   callback            Invoked when the query is completed
 * @param  {Object}     callback.err        An error that occurred, if any
 * @param  {Row}        callback.row        A Helenus Row object, holding just the columns that were selected in this "page"
 * @param  {Boolean}    callback.truncated  Whether or not any column was truncated from the results
 */
var runPagedColumnQuery = module.exports.runPagedColumnQuery = function(columnFamily, keyColumnName, keyColumnValue, start, limit, opts, callback) {
```

This method handles all current paging needs in the application, and replaces the `getPagingParameters` method. I think this new method should be easier to use. I wrote it while scoping out bootcamp-like projects, and realized that any project will probably need paging, and the current technique for paging is too hard to talk about.