Add documentation for Dates types
Add cross-references for "Calling C and Fortran code"
Add function and literal crosslinks for "Complex and Rational Numbers"
Cross-link "Constructors"
Cross-reference "Control flow"
Cross-reference "Conversion and Promotion"
Cross-reference "Dates"
Convert dates codeblocks to doctests

NOTE: Currently, the doctests fail, presumably because of some change in functionality?

example:

Failed example:
    Dates.tofirst(istuesday, Date(2014,7,13); of=Dates.Year)
Expected:
    2014-01-07
Got:
    ERROR: `tofirst` has no method matching tofirst(::Function, ::Date)
Redefine Dates.recur signature

This change removes sphinx's confusion by the type parameter in function
signature