Remove interpolate() method that shouldn't be used
SplineInterpolate is now parametric type

Will allow flexibility in choice of x, y type. In particular, will let
one define interpolation on Real or TimeType or whatever. Will need to
write interpolate() and calibrate() methods to cater for this.
interpolate() method dispatch on specific SI type

Dispatches on SplineInterpolation{Real, Real}. Dispatch on other
concrete SplineInterpolation types will not work and probably should
not work as the coefficients field is a matrix of Real and hence x, y
are naturally Real.
calibrate() for x::TimeType, y::Real
Minor code style change
Added RealSplineInterpolation typealias
Use RealSplineInterpolation in method signatures
Added interpolate() for TimeTypes and...

fixed calibrate(T<:TimeType…) method
years() to return float when dates equal
Remove interpolate() test for 881a9d5
Import Base.zeros

This shouldn’t be necessary
years() to return float

improves on 5ad52f2. Also separated methods for Date and DateTime type
inputs. Will need to improve documentation
ZeroCurve constructors and methods

Still testing. Having issues with the last expression in
test_yieldcurve.jl:

```
ERROR: `calibrate_cubic_spline` has no method matching
calibrate_cubic_spline(::Array{Float64,1}, ::Array{Real,1},
::SparseMatrixCSC{Float64,Int64}, ::Array{Any,1})
 in calibrate at
/Users/imanuel/.julia/v0.3/FinancialMarkets/src/maths/interpolators.jl:1
28
 in ZeroCurve at
/Users/imanuel/.julia/v0.3/FinancialMarkets/src/pricingstructures/yieldc
urve.jl:59
 in ZeroCurve at
/Users/imanuel/.julia/v0.3/FinancialMarkets/src/pricingstructures/yieldc
urve.jl:31
```