Added options for Skrill gateway

Added possibility to:

* add a custom payment icon
* add a custom logo til skrill
* add a string for the type of payment you would like to use at skrill
Validate URL + more

Thanks!

1). I added the description to the URL field as you said

2). I'm validating the URL when saving to DB now with the
FILTER_VALIDATE_URL but I'm not showing any issues in the GUI. I could
not find where to do that. Any help here?

3). I remove the birthday field for now. The user will have to fill
that out to Skrill instead.

4). I added translation to the confirmation note.
translation for confirmation_note

5). Yes. That would be great. I started there but there are so many
different options that I'm not sure if I'm able to pull that of. I
don't know which are allowed and not allowed together. Look at p40 here
https://www.moneybookers.com/merchant/en/moneybookers_gateway_manual.pdf
 Do you have any other idea? That is not that hard for me to implement?
Added multiselect for skrill payment options

I added four of the credit cards. I'm still unsure if this is the
correct way to do it.

Do you think we should add any more?
Translated the credit card types
Merge pull request #790 from olind/dev

Added options for Skrill gateway