Replaced RASqliteRow with categories for NSDictionary and NSMutableDictionary.

The `RASqliteRow` class have been replaced with categories for both `NSDictionary`
and `NSMutableDictionary`, where the mutable version is (and should) only be
accessable from the `RASqlite` class, hence import from the implementation file.

The categories implement a more consistent way of handling `nil` values. Where
the dictionaries would throw an exception when assigned a `nil` value the
categories will assign `NSNull null` instead.

The category methods should always be used instead of the default dictionary
methods for setting and retrieving objects.

	`setObject:forKey:` have been substituted with `setColumn:withObject:`.
	`objectForKey:` have been substituted with `getColumn:`.

Also, an additional method (`hasColumn:`) have been implemented to easier and
more intuitive check if the dictionary has a key.

The sample code and readme file have been updated.