Adds support for the ``.. code::`` rst directive.

This modifies the existing ``.. code-block::`` to be treated as spec
extension with a specific default language syntax. Also implements line
numbering.
Modifies docgen to support new warning message.
Improves macro tutorial with hyperlinks and numbered lines.
Hyperlinks quit related docstrings in system module.
Hyperlinks slurp related docstrings in system module.
Handles :file: option for code blocks to avoid warnings.
Add list comprehensions to future module
Extend list comprehension documentation
Add -lm for fesetround and fegetround
Move floating point rounding and exceptions handling to math
(it's C99 as well, not just POSIX)
Preserve nil <-> NULL between Nimrod and database.
Move fenv to its own module
Merge branch 'devel' of github.com:Araq/Nimrod into devel

Merging mainline devel.
Merge branch 'devel' of github.com:Araq/Nimrod into devel

Merging mainline devel.
Merge branch 'devel' of github.com:Araq/Nimrod into devel

Merging mainline devel.
Tweaks for postgres driver (not tested yet).
Fixes line numbering.
Documents json module.
Hides TJsonError, it wasn't being used.
Adds stringification support for nnkPostfix nodes.