Add a --force option to `mix local.install`.

Useful for makefiles.
Added more informative error to Makefile when ex_doc is not available.

Added more information to README about building docs.

fixes #1151
Merge pull request #1221 from alexrp/mix-local-install

Add a --force option to `mix local.install`.
Merge pull request #1222 from jjh42/doc-error

Added more informative error to Makefile when ex_doc is not available.
Rename test/doctest.exs file to test/doc_test.exs to follow file name convention
Fix Makefile dependencies
Add missing rm -rf ebin back to docs task