Allow capitalized `Serialize` functions

Currently cereal searches for `serialize`, `load` and `save`. If they could be made `#define`s, the user of the library could specify the name of the functions that cereal should be looking for.

```c++
#ifndef CEREAL_SERIALIZE_FUNCTION_NAME
#define CEREAL_SERIALIZE_FUNCTION_NAME serialize
#endif
#ifndef CEREAL_LOAD_FUNCTION_NAME
#define CEREAL_LOAD_FUNCTION_NAME load
#endif
#ifndef CEREAL_SAVE_FUNCTION_NAME
#define CEREAL_SAVE_FUNCTION_NAME save
#endif
```

This is because some [styling](http://msdn.microsoft.com/en-us/library/aa260844#cfr_names) [guides](http://google-styleguide.googlecode.com/svn/trunk/cppguide.xml) require functions that are `Capitalized` and having `Serialize`, `Load`, `Save` would conform.

How do you guys feel about this? Is this something that would be acceptable to create a pull request for?