Staging the infra of a basic PDE Solver

See the discussion at https://groups.google.com/forum/?fromgroups=#!topic/sympy/bBenTf1lG70 . 

And also point 2 under theory at https://github.com/sympy/sympy/wiki/GSoC-2013-Application:-Manoj-Kumar-:-Improved-ODE-Solver . I thought this could be implemented in ode.py itself, but then according to the discussion, extending the pde solver to these type of simple equations, could be beneficial.

So I decided to give this a shot myself. The code borrows its architecture, heavily from the ode.py code and for now I've added a hint that solves simple linear homogeneous partial first order PDE's.

However I've not added an ' integral' , 'best' or 'all' hint since, I've implemented only a 
single hint till now that doesn't need actual integration. Also I've yet to add a checkodesol equivalent yet.