Added function to free hidden internal GL context to avoid memory leak

After working on a memory leak in SFML.NET I have narrowed the issue down to SFML creating many contexts that are never destroyed. The following code example is enough to cause SFML to create contexts and leak memory.

```
#include <thread>

#include <SFML/Window.hpp>

void ThreadEntry()
{
    sf::Context(); // create explicit context - when this is destroyed
//SFML automatically creates a hidden context, the same would happen if you loaded a texture
}
int main()
{
    sf::Context mainThreadContext; // create a main context to avoid SFML's cleanup functions - same
//as if a window was active in this thread
    while (true)
    {
        std::thread(&ThreadEntry).join();
    }
}
```

Per the following thread, @LaurentGomila explains that SFML internally creates a hidden internal context for each thread that touches any of SFML functions that require a valid GL context. As you continue reading the thread, a suggestion was to add a public function called "releaseThreadContext" to release this internal context. This is what this PR does.

http://en.sfml-dev.org/forums/index.php?topic=10309.msg70990#msg70990


Now, just for kicks I followed up another thread post made by @binary1248.

http://en.sfml-dev.org/forums/index.php?topic=13568.msg95089#msg95089

The code example here is to show that it possible that memory leaks may also come from the video driver. I tested the example code and it does not leak, it also does not leak for @rcurtis (whom also experienced the memory leak with SFML not freeing contexts). So this pretty much rules out drivers leaking.

This pull request only adds a single public function ```sf::Context::releaseThreadContext()```. After modifying the above example code to the following all memory leaks are gone.

```
#include <thread>

#include <SFML/Window.hpp>

void ThreadEntry()
{
    sf::Context();
    sf::Context::releaseThreadContext(); // dispose the hidden context that was created
//when any SFML functions are called
}
int main()
{
    sf::Context mainThreadContext;
    while (true)
    {
        std::thread(&ThreadEntry).join();
    }
}
```