Snowhouse v3.0.0

This is a compatibility-breaking release of Snowhouse.

The main (user-relevant) changes in this release are:

• A bug is fixed that AssertThat did sometimes not work while Assert::That did (see #26).
• Our macros that behave like functions, like AssertThat or AssertThrow, must be followed by a semicolon.
• Several problems arising with C++11's nullptr_t and our IsNull are resolved (see #17 and #18).
• Stringizers are officially documented (see #20).
• Container stringizers are smarter and handle every container having a const iterator as well as begin() and end() methods.
• IsEmpty() and Is().Empty() are using the containers' empty() method instead of size() (see #14). This extends this functionality to work with STL containers that have no size() method, like C++11's std::forward_list.
• Snowhouse is now explicitly using Semantic Versioning 2.0.0.