File Open modes: Use an real enum instead of const values

This will also silence compiler warnings about sign mismatches
include/pandora.hpp: Add guards around enum
Tests: Split tests in .cpp and .hpp and add a TestAll binary

The TestAll binary runs all default tests. make check will invoke
only the TestAll binary.
XCode: updates
Merge pull request #7 from gicmo/test_all

Create a "TestAll" test that contains all other tests and is run by default
Merge pull request #6 from gicmo/file_modes

File Open modes: Use a real enum instead of const values