Error tuples

[GeoffreyPS]

This PR corresponds to issue #23 (Error Conventions)

If merged, this commit will:

• change returns of most functions from result to either {:ok, result} or {:error, reason}.
• functions that are composed of other Statisaur functions have slightly different internal implementations, typically leveraging the with macro, but may use try/rescue blocks in rare instances.
• add bang versions of most functions (e.g. adds median/1 and median!/1).
• add new test cases for error testing; alter test cases to correspond with new return values.

These changes extend to the original Statisaur module and Statisaur.Combinatorics module, as they are the only modules on the master branch.

Please let me know if there are other actions I need to take before this can be merged (bumping version number, fixing anything, etc). Thanks for taking the time to review!

[GeoffreyPS]

I just remembered that this will also require use of Elixir version 1.2 or higher with use of the with macro in the implementation of some functions. After discussion, I can go ahead and bump the required Elixir version within mix.