Normalise all paths in _find_file()

Fixes the original issue in #28.
Don't cache filename lookups in _include()

because the same include/require argument may refer to different files.

Also, restore the original global_frame when coming back from
an include at global scope.
Move find_file() from BytecodeCache to Interpreter

Include files relative to the current code file's directory,
instead of the top-level script's directory.
Fixes test_phpt/lang/include_variation2.phpt
Fix semantics of require/include

Don't lookup the include path if the requested name starts with
a dot or a slash.

Closes #28
Update conftest
Merge pull request #52 from hippyvm/issue28

Issue28