Add info error level

Add new error level `info` for informative messages.

Requires #212.

# Rationale

Some syntax checkers do indeed report useful informative messages.

Users may want to define their own syntax checkers with informative messages.

# Implementation

- Define a new face for info level highlights.  Check out `flymake-infoline` from the Flymake fork
- Find a suitable built-in fringe icon, or define a new one.
- Define the new error level with `flycheck-define-error-level`.
- Determine which built-in checkers need to use the new level

# Status

- [x] Define the new face
- [x] Define the fringe icon
- [x] Define the error level
- [x] Update built-in checkers
- [x] Document the new level
- [x] Add unit tests
- [ ] Notify Solarized and Zenburn themes about the new face

# Affected checkers

- Clang (for `note` messages)
- Pylint (for convention messages, code `C`)
- …?