Added convenience string to date and date to string functionality in QL Date class.
Merge remote-tracking branch 'lballabio/master'
Merge remote-tracking branch 'upstream/master'
Merge remote-tracking branch 'lballabio/master'
Merge branch 'master' of https://github.com/mdriesch/quantlib
Moved parsing of dates to existing utility class.

Previous parsing functionality was deprecated. A few tests were added.
Renamed holder generator to "formatted_date".

It seems more descriptive and less ambiguous than "userdefined_date"
(it's the format which is user-defined, not the date).

Also, the preprocessor checks for Boost > 1.33 were removed (since
we already assume that Boost is at least at version 1.34.1).
Merge pull request #29.