Added GMRES to solvers.jl

GMRES implementation I've been developing. I'm fairly new to Julia so there maybe some changed needed to make the code more "Julianesque". In particular, I wonder about the way I'm handling named arguments and the optional left and right preconditioners (Mr and Ml). In MATLAB, you could check nargin or would check isempty(Mr), but I thought this approach was closer to idiomatic.
Added GMRES test on SPD matrix
Fixed error in caculating tolerance with right preconditioner
Export gmres
Support for sparse matrices

Changed type of A from Array to AbstractArray
Added support for function inputs, simplifications
Update KrylovSolvers.jl
Changes based on user group discussion
Update KrylovSolvers.jl
Update README.md
Merged pull request #1 from priesj/patch-1
gmres: changed restart into named arg, replaced view assign. with fill!
Merge branch 'gmres' into develop