Avoid use this(window) on WebSqlProxy

[dukex]

The src/windows/WebSqlProxy.jsuse this to create a db var, but this is window and db is very common word to make reference a database opened, for example in README example:

var db = openDatabase("Todo", "", "Todo manager", dbSize, function() {
    console.log('db successfully opened or created');
});

Other example is when project has the db.js as dependency, the db.js use window.db too.

Using the module.exports as in cordova-plugin-file the db in WebSqlProxy is scoped in module

[vladimir-kotikov]

@dukex Thanks!