make default CMAKE_BUILD_TYPE == RELEASE

This behaviour was lost when giving up the "seqan build system".

Add:

``` cmake
if (NOT CMAKE_BUILD_TYPE)
    set (CMAKE_BUILD_TYPE Release CACHE STRING
         "Choose the type of build, options are: Debug Release RelWithDebInfo ..."
         FORCE)
endif (NOT CMAKE_BUILD_TYPE)
```

see also #70 
