Solving first order linear general PDE's with constant coefficients

This continues my work of the previous PR. Now these type pf PDE's can also be solved.

    >>> from sympy.solvers.pde import pdsolve
    >>> from sympy import Function, diff, pprint, exp
    >>> from sympy.abc import x,y
    >>> f = Function('f')
    >>> eq = -2*f(x,y).diff(x) + 4*f(x,y).diff(y) + 5*f(x,y) - exp(x + 3*y)
    >>> pdsolve(eq)
    f(x, y) == F(4*x + 2*y)*exp(x/2 - y) + exp(x + 3*y)/15

**TODO**

- [x] Fix exception message
- [x] Give general solution in docstring
- [x] Fix the solution to make sense (what is `E`?)
- [x] Fix math formatting and grammar errors in docstring