Added overall better API coverage, ability to delete keys, and ability to bulk read/write

Updated README.md with examples
Added ability to delete preference keys
read() function now accepts either a single key or a list of keys
write() now accepts a key/value or a dictionary of key/value pairs
Preferences object now defaults to kCFPreferencesCurrentUser and kCFPreferencesAnyHost, which equates to ~/Library/Preferences/*.plist
Preferences object now uses CFPreferencesSynchronize instead of CFPreferencesAppSynchronize
Preferences object now has internal methods set, get_multiple, set_multiple, and delete_multiple
Preferences object now accepts host as a parameter