[Withings] Enable multiple accounts

The main change lies in parsing the openhab.cfg file differently. One can simply add the needed parameters like this

```
withings-oauth:userid=4866592
withings-oauth:consumerkey=74c0e77021ef5be1ec8dcb4dd88c1xckusadwe92f9541c86799dcbbfcb8fc8b236
withings-oauth:consumersecret=25f1098209xns511711b3287288f90740ff45532cef91658c5043db0b0e0c851c
withings-oauth:token=e587df2b08fec066c7643081c649e8517987f6c25d8089d4230987399e28d6
withings-oauth:tokensecret=7c16e5f2f1f5fd784211bfff7c1435jh2d9419e503c5d079e114719b66dcef9
```
or additionally add an user id to which this credentials belong to like:

```
withings-oauth:thomas.consumerkey=74c0e77021ef5be1ec8dcb4dd88c1xckusadwe92f9541c86799dcbbfcb8fc8b236
withings-oauth:thomas.consumersecret=25f1098209xns511711b3287288f90740ff45532cef91658c5043db0b0e0c851c
...
withings-oauth:peter.consumerkey=74c0e77021ef5be1ec8dcb4dd88c1xckusadwe92f9541c86799dcbbfcb8fc8b236
withings-oauth:peter.consumersecret=25f1098209xns511711b3287288f90740ff45532cef91658c5043db0b0e0c851c
```

Furthermore: if there were no values stored into openhab.cfg yet the Binding store it's values into the file services/withings-oauth.cfg which goes into the ESH way of storing config values. Since this specific file is only meant to configure withings values no prefix is needed. An example file look like:

```
thomas.consumerkey=74c0e77021ef5be1ec8dcb4dd88c1xckusadwe92f9541c86799dcbbfcb8fc8b236
thomas.consumersecret=25f1098209xns511711b3287288f90740ff45532cef91658c5043db0b0e0c851c
...
peter.consumerkey=74c0e77021ef5be1ec8dcb4dd88c1xckusadwe92f9541c86799dcbbfcb8fc8b236
peter.consumersecret=25f1098209xns511711b3287288f90740ff45532cef91658c5043db0b0e0c851c
```

Best, Thomas E.-E.
I guess the source path can be removed.