[borgar]

Without a locale, every function now uses the built-in en locale.

Before, the defaults disagreed. parseValue used en-US, while parseDate, parseNumber, parseTime, and parseBool did not. format() used hard-coded patterns for the system date and time codes.

• defaultLocale is now the built-in en locale, so unknown locale tags also fall back to en.
• getLocale() accepts undefined and an empty string, and returns undefined for them.
• delocalize() falls back to en for an unknown locale.

A side effect to this that I had to fix is that:

A/P prints A or P in every locale

A/P printed a single letter only when no locale was given. With any locale, including en-US, it printed the locale's full AM/PM names. It now always prints A or P, as Excel does.

A driveby fix: Regexp matcher for TOKEN_MODIFIER was incorrect. This doesn't affect much since formats that this happens in would have yielded errors anyway.

Fixes #97