* Integrated filesystem operation abstraction code into filesystem.cpp
* Modified filesystem unit tests to be more platform agnostic
* Added append mode for PlatformFile
* Minor bug fixes in filesystem operations
Some of the types in fileops tests were causing warnings to be thrown
during build, due to type mismatch. I've added a few local variables to
quiet these warnings.
* Implemented filesystem operations abstraction code
* Added filesystem operations abstraction unit tests
* Modified CMake configurations to support the building of the abstraction code and unit tests
