parser default implementation loading

Using ServiceLoader to load the default Parser implementation
Using java standard Logger (use slf4j?)
Added a test checking that an exception is thrown if invalid escape
sequence in String.
BuiltinTomlParser should be thread safe.
renamed BuiltinTomlParser to SimpleTomlParser

renamed `BuiltinTomlParser` to `SimpleTomlParser`
fixed #9

Fixed issue #9: Should getInt() be getLong() ?
Overwriting previous keys is forbidden

Overwriting previous keys is forbidden and produces an exception.
Overwriting previous keys is forbidden
Overwriting previous keys is forbidden