Add sync with Splitwise

[sanekbel]

Beta version of synchronization plugin for Splitwise (https://splitwise.com).
Input your API token from https://secure.splitwise.com/apps.

    This plugin synchronizes your Splitwise expenses with ZenMoney.

    ## How it works

    1. The plugin creates separate accounts for each currency used in your Splitwise expenses
    2. All expenses where you owe money will be imported as transactions with negative amounts
    3. All expenses where others owe you money will be reflected in the account balance

    ## Transaction handling

    When you pay for a shared expense:
    1. The original payment will appear in ZenMoney from your payment method (card/cash)
    2. After syncing with Splitwise, your share of the expense will appear as a transaction
    3. You should convert the original payment to a transfer:
       - Source: Your payment account (card/cash)
       - Destination: The corresponding Splitwise currency account

    This way, your Splitwise balance will accurately reflect what you owe and what others owe you.

    ## Example

    You paid $100 for dinner with a friend (split 50/50):
    1. Original $100 payment appears from your card
    2. Splitwise sync adds a -$50 expense (your share)
    3. Convert the $100 card payment to:
       - Transfer from: Your card
       - Transfer to: Splitwise USD account

    Final result:
    - Your card: -$100
    - Splitwise USD: +$50 (friend owes you)