Raise ``ValueError`` for ``gen_string`` function.

If an invalid string type is requested from ``gen_string``, raise
``ValueError`` instead of a generic ``Exception``. Fixes issue #30.
Merge pull request #46 from omaciel/fix-issue-30

Raise ``ValueError`` for ``gen_string`` function.
Cleanly separate readme from full documentation

Make the readme a real file, instead of a link to `docs/index.rst`. Give the
readme a summary of the project, and include instructions for finding the rest
of the documentation.

Remove the non-local image references from `docs/index.rst`. This silences the
remaining Sphinx warnings.
Merge pull request #48 from Ichimonji10/docs

Cleanly separate readme from full documentation
Ready for 2.0 release.