Add tests from Mustache specification v1.1.2.

First of all, thanks a lot for qt-mustache! I'm using it in [protoc-gen-doc](https://github.com/estan/protoc-gen-doc) and love how small it is.

This pull request adds a new test `TestMustache::testConformance()`. The test is driven by JSON data from version 1.1.2 of the [Mustache spec](https://github.com/mustache/spec), which has been copied to the `tests/specs` directory. The test is only built when Qt 5.0.0+ is available, since it uses the JSON classes.

I've tested that building with Qt 4.8.6 still works.

Unfortunately qt-mustache fails about half of the tests :( To see the failures, uncomment the `QEXPECT_FAIL(..)` calls in the test. I haven't looked into fixing any of these failures, but I think that at least some of them shouldn't be too hard.

Personally I'm mostly interested in getting the issues with standalone tags sorted.