Crsf protection test
Initial CSRF protection
Minor documentations edits in ErrorHandled, Exceptions and Router
Clean out the code for Plug.Head

Took advantage of pattern matching in order to clean up the code in `Plug.Head`
(by matching directly on `conn.method`).
Merge pull request #142 from whatyouhide/minor-polishes

Minor polishes to some docs and to Plug.Head's code
Update the format in mix.lock

No more `:package`, now Hex uses `:hex`, `:git` and similar "tags".
Merge pull request #143 from whatyouhide/update-mix-lock-format

Update the format in mix.lock
Fix plug builder error message
Skip csrf plug when plug_skip_csrf_protection private is true
For Javascript GET requests, allow only XHR reqs
Merge pull request #136 from gjaldon/csrf-plug

CSRF plug
Improvements to CSRF

* Always set the CSRF token in session
* Use Plug.Crypto.secure_compare/2 to compare tokens
* Rename "authenticity" to "csrf" throughout
* Check for cross-origin regardless of the accept header
Release v0.9.0
Minor docs enhancements and code formatting
Merge pull request #144 from whatyouhide/docs-enhancements

Minor docs enhancements and code formatting
Update README.md