JCLOUDS-427: Avoiding Guava reflection code broken in Java 7u51

- Adding a version of TypeToken.where that replaces two parameters in one go
- Avoiding TypeToken type parameter replacement with wildcard bounds

Works around https://code.google.com/p/guava-libraries/issues/detail?id=1635

Backport to 1.6.x of #261. **Also upgrades 1.6.x to Guava 15.0**, because classed the workaround needs to access are not accessible in 14.x. **Is that OK?**