New module XrdFileCache for disk based file cache

The pull request contains new module implemented in src/XrdFileCache. In addition there are changes in the following subdirectories:

* Doxyfile :  add Doxygen documentation for XrdFileCache sources

* src/CMakeLists.txt : build new XrdFileCache plugin

* packaging/rhel/xrootd.spec.in : add libXrdFileCache.so to list of server libs

* XrdPosix : src/XrdPosix/XrdPosixFile.cc in Finalize() -- Stat() the file before creation of the cache objects as they need to know the size of the file.