Add support for SQL Standard PERCENTILE_CONT() and PERCENTILE_DISC() inverse distribution functions

[lukaseder]

Documentation:

• PERCENTILE_CONT(): http://msdn.microsoft.com/en-us/library/hh231473.aspx
• PERCENTILE_DISC(): http://msdn.microsoft.com/en-us/library/hh231327.aspx

Interesting to note that SQL Server adopted Oracle's WITHIN GROUP (ORDER BY ...) clause, which we might see again for ordered aggregate functions.

These are also available in PostgreSQL 9.4 (see #3871) and are a prerequisite for #3872

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Note: Interestingly, SQL Server seems to support these only as window functions, whereas other DBs support them only as aggregate functions!