gitweb.js: Extract and improve datetime handling

gitweb.js: Extract and improve datetime handling

Move formatDateISOLocal(epoch, timezone) function (and also helper
timezoneOffset(timezoneInfo) function it requires) from common-lib.js to
datetime.js

Add new functions:
* localTimezoneOffset - to get browser timezone offset in seconds
* localTimezoneInfo   - to get browser timezone in '(+|-)HHMM' format
* formatTimezoneInfo - turn offset in hours and minutes into '(+|-)HHMM'
* parseRFC2822Date - to parse RFC-2822 dates that gitweb uses into epoch
* formatDateRFC2882 - like formatDateISOLocal, only RFC-2822 format

All those functions are meant to be used in future commit
'gitweb: javascript ability to adjust time based on timezone'

An alternative would be to use e.g. Datejs (http://www.datejs.com)
library, or JavaScript framework that has date formatting (perhaps as
a plugin).

While at it escape '-' in character class inside tzRe regexp, as
recommended by JSLint (http://www.jslint.com).

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>