Fixed bug in DBi::get()

Bug mainly happened when DBi::add() tried the set the current
connection and array_key_exists in DBi::get() couldn’t work because the
passed argument was connection and not a string or integer.
Improved features in README.md
Commented private class variables and constants
Closes #13, changed default unit of $duration to milliseconds

Added a second argument in getDuration() for returning the duration in
seconds.