Add bivariate cumulative student t distribution.

Thanks to Michal Kaut.

Closes #121.
Fix file names in project filters.
fix some garbage / never used values, add infinity
Export monotonic log-discount curve.

Also, add a few more traits for cubic interpolations.
Add Vecer engine for continuous-averaging Asian options.

Thanks to Bernd Lewerenz.
Fix inflation-rate interpolation.

Thanks to Amine Ifri for the heads-up.
revert definition of QL_INFINITY (which may be misleading if has_infinity() is false), replace minus infinity by minus maximum real for log 0 (what really matters is only to have a defined value here)
Merge pull request #173.
Refresh documentation page.

- add link to videos from Felix Lee;
- move blog list more to the front;
- group conference slides under a single heading.
Merge pull request #2 from lballabio/master

get new code