Named parameters support (breaking)

Included in this release:

• Syntax: Added support for named parameters via name=value syntax.
• ShowDate Command: timezone: keyword converted to a named parameter.
• SetVar Command: count: keyword converted to a named parameter.
• SetVar Command: Added setCount().
• For Command: break-at: keyword converted to a named parameter.
• For Command: Added setBreakAt().
• ShowDate Command: Fixed incorrect conversion of some PHP date format signs.
• ShowDate Command: Added a, G and g to the formats for AM/PM times.
• ShowDate Command: Added tests for all format conversions.
• ShowDate Command: Added constants for all PHP format signs.
• ShowDate Command: Added setTimezone().
• Error Messages: Added getCommand() to fetch the originating command, if any.
• Factory: Added the var() set to access variable related methods.

Breaking changes

Some keyword constants were removed because they have been replaced
with named parameters:

• TYPE_TIMEZONE
• TYPE_COUNT
• TYPE_BREAK_AT