Spelling fix
implement normalization constant for discrete truncated power law

The directly computable normalization constant significantly reduces
the runtime for fitting a discrete truncated power law.

For a discrete truncated power law of form f(x) = x^-a*e^(-l*x), the
normalization constant C such that that sum of C*f(x) from x=xmin to
x=Infinity equals 1 is
   C = e^(l*xmin) / Phi(e^-l, a, xmin),
where Phi is the Lerch transcendent (aka Lerch Phi) function.
Merge pull request #3 from drbild/add-normalization-for-discrete-truncated-power-law

implement normalization constant for discrete truncated power law
Pulling from Github.
Added xmax case to discrete PDF normalizing factor for truncated power law and
    stretched exponential.
Set discrete PDF normalizing factor property from the Distribution
class to False, so it won't exist unless explictly created. The method
for calculating it directly from CDFs, which was implemented, only works
    if you have discrete CDFs, which is presently only the case for the
        power law.