Introduce new account file structure.

This changes the locations that account files (wallet.bin, utxo.bin,
and tx.bin) are searched for when opening or disk syncing accounts.
Previously, files were saved in the following layout:

  ~/.btcwallet/
    - btcwallet/
      - wallet.bin
      - tx.bin
      - utxo.bin
    - btcwallet-AccountA/
      - wallet.bin
      - tx.bin
      - utxo.bin

This format had two issues.  First, each account would require its own
directory, causing a scalability issue on unix (and perhaps other)
platforms.  Second, there was no distinction between testnet and
mainnet wallets, and if mainnet support was enabled, btcwallet would
attempt to open accounts with testnet wallets.

Instead, the following file structure is now used:

  ~/.btcwallet/
    - testnet/
      - wallet.bin
      - tx.bin
      - utxo.bin
      - AccountA-wallet.bin
      - AccountA-tx.bin
      - AccountA-utxo.bin

This solves both of the previously-mentioned issues by requiring only
two subdirectories (one each for the testnet and mainnet bitcoin
networks), and by separating the locations to open and save testnet
and mainnet account files.

At startup, a check for the old account file structure is performed.
If found, files are moved to the new locations, and the old account
directories are removed.  Account files are moved to the testnet
directory, as only testnet support is currently enabled.

The version has been bumped to 0.1.1 to reflect this change.

Fixes #16.