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
Fix code escaping in ExUnit.Callbacks docstring
Provide reference documentation for module attributes
Document `__info__/1`
Add support for `h AnyModule.__info__`. Closes #1230
Merge pull request #1229 from alco/1132-doc-module-attr

Provide reference documentation for module attributes
implement mix package and related tasks re #1124
changes from PR discussion

1: ensure_loaded?

3: ebin

2: assume path/ebin to be packaged

6: accept project path & assume ebin

8: include priv in archive

5: local.uninstall handles packages too

7: local.install [] will install a package

7: mix new .gitignore *.ez files