Provides cg (conjugate gradients)
Merge pull request #8 from JuliaLang/pr/3

Provides cg (conjugate gradients)
Refactors conjugate gradients

cg now:
- uses nonzero default guess for x
- throws PosSemidefException if matrix is not positive semidefinite
- returns solution vector with ConvergenceHistory
- no longer prints anything
Duck-types KrylovSubspace. (#2)
Provides ModifiedGramSchmidt (= Householder) as the new default orthogonalization method
Complete duck-typing of simple and Lanczos methods
- Removed unnecessary specializations of parametric types.
- Uniformly implemented ConvergenceHistory returns
- Renamed loop iterator to iter instead of k
- General cleanup
- XXX Commented out failing tests!
Merge branch 'master' of git://github.com/priesj/IterativeSolvers.jl into priesj-master