Add syntax checker for Sphinx
Merge branch 'sphinx'

* sphinx:
  Add syntax checker for Sphinx
Improve flycheck-valid-checker-p

Also make sure that we have a symbol
Add a function to validate a next-checker cell
[Fix #266] Add next checkers to syntax checkers
Merge branch 'extend-syntax-checkers'

* extend-syntax-checkers:
  [Fix #266] Add next checkers to syntax checkers
  Add a function to validate a next-checker cell
  Improve flycheck-valid-checker-p
Explain the use of pipes more verbosily
[Fix #298] Do not attach the checker process to the buffer

Avoid any side effects from linking processes to buffers.  We don't need
these, we just need to know the source buffer, so we get away with
storing the buffer in a process property.
Fix test regression from f0ab109
Restore compatibility with ESLint 0.3.0

Since 0.3.0 the "stylish" formatter is the default. "compact" is still easier parseable, so force it.
Update test cases for ESLint 0.3.0