Added a Singleton VaultHolder with weak references to Vaults
This should allow the app to share a Vault over different views without
re-creating the vault with the expensive PBKDF2 operation.

It is crucial, that the VaultHolder is cleared when leaving a Vault,
otherwise this might still be accessible without the correct password.

This concept has to be reviewed!
Modified VaultHolder to hold only one Vault.
The currently loaded Vault can be retrieved without the name or password.

Every invocation with name and password clears previously loaded Vaults for
security reasons.
Merge pull request #27 from L-Henke/vaultHolder

Added a Singleton VaultHolder with weak references to Vaults