Updated .gitignore

Project files and files generated during compilation were added to the
list of stuff to avoid to be tracked by Git.
Created POSIX library

The required files to build the POSIX system was added to the library.
Added the error header file for the POSIX system
Added a constructor to SyscallError
Added errno code information

A method to get the numeric value of the error reported in the errno
global variable was added to the class.