Reading preference files
preference validation functions
Replaced old preferences stuff throughout Brian, including sphinxext
Forgot to put in previous commit: added core.core_preferences file,
where you should define the core preferences
Added tool to automatically generate default_preferences file, should be
run before any new release
Allow file object instead of filename in read_preference_file
Various changes to preferences
* Changed preferences to use values rather than strings
* put everything into BrianGlobalPreferences
* changed validation functions (default validators are back)
* derived BrianGlobalPreferences from MutableMapping
Warning on Network.run if unvalidated preferences set
Added preference tests and fixed a bug
Some minor docstring and style changes, removed unused imports.
Merge pull request #6 from brian-team/preferences

new preference system, fixes #5