make ex_doc only-dev dep

Hello!
Your project looks neat :)

Now from my experience this little change is going to make life easier.

Generating a documentation is usually task you do on your dev machine. It's not necessary to carry this package as a prod subdep when someuse uses your package as a dep.

Also, a single git dep can screw deps resolution - again, this is usually not hurting when you're inside *this* project, but starts to be annoying when people grab your code from Hex as a dep.
merging pull request #1
added math helpers module
placeholder module and test
added placeholder functions
created the cdf function
added the cdf function (without size parameters)
nicer 'ternery-ish' expression
added location/size parameters to cdf function
added tests to the math helpers module
moved all the functions from the Statistics.Descriptive module into Statistics module; Descriptive module removed
added random number draw from distribution
added ppf function
added ppf function
added location/scale to ppf function
updated readme
fixed random number generation
added docs url to readme
enable less-precise tests to mitigate rounding errors
extra rounding test case