Fix Class Not Found Exception 

Android doesn't have AuthCache as part of its http client library, to get around this the authentication header is being added manually to each request if needed
Pre-calculate the header value in the [constructor](https://github.com/cloudant/sync-android/pull/30/files#diff-73d3dc1327eefa658c82d888916c836fR88) and store it as an instance variable. Doing the base-64 encoding for every request is expensive.