Add SQLCipher support

SQLCipher support has been added as a subspec, and it is very easy to setup. Following fmdb's example, just change your Podfile to:

    pod 'YapDatabase/SQLCipher'

If you aren't using the SQLCipher subspec, your project won't even compile the encryption configuration options to prevent the case of accidentally trying to use encryption when support is not available.

```obj-c
YapDatabaseOptions *options = [[YapDatabaseOptions alloc] init];
options.corruptAction = YapDatabaseCorruptAction_Fail;
options.passphraseBlock = ^{
    // You can also do things like fetch from the keychain in here
    return @"super secure passphrase";
};
    
self.database = [[YapDatabase alloc] initWithPath:databasePath
                                 objectSerializer:NULL
                               objectDeserializer:NULL
                               metadataSerializer:NULL
                             metadataDeserializer:NULL
                                  objectSanitizer:NULL
                                metadataSanitizer:NULL
                                          options:options];
```

I implemented the passphrase as a block to help prevent storing the credentials in memory any longer than necessary. This block will be executed on database setup, and when new connections are made to the database.

This pull request resolves issue #4.