Fixes to auto-config search paths (bugs 228 & 229).

1) Interpreter file names must now match a regex pattern to be valid
(as opposed to just starting with a certain string).

2) Certain interpreter name patterns can be given higher priority
over others (so that Quick Auto-Config can choose the system default
Python over all others, for instance).
Auto-config performance & priority improvements, and make Pypy discoverable.

1) Preserve the order of paths in PATH when they are searched through by the
auto-configurer, in order to prioritize discovered interpreters by the order
of their parent directory's appearance in PATH.

2) Allow the Python auto-configurer to search the PATH.

3) Allow Pypy interpreters to be discovered by the auto-configurer.