Improve the documentation

* Add an email address so that developers may notify us of new modules
  they may have developed.
* Add documentation on the use of Erebot_Interface_HelpEnabled.
* Add a note about the pthreads extension on the subject of PHP's
  multithreading support.
* Add Symfony's YAML component's licence to already listed licences.
* Improve display of supported modules (using multiple columns).
* Use the new "erebot:module" domain to make the docs more compact.
* Update the prerequisites page:
  * Add a glossary
  * Update windows instructions regarding gettext
  * Use named footnotes to improve readability
  * Fix some of the links for pecl extensions
* Remove references to PEAR on index page and add new badges.
Work on the Windows installer (#29).

Move the sources from contrib/nsis to windows folder.
Add an annotated Erebot.xml as a sample for new installations.
Fix several paths in .nsi script.
Add a new "release_win" target.

This target produces the Windows installer for Erebot.
For the time being, this target only works on Linux due to the use of
environment variables in the command path.
Do not restrict travis-ci to the master branch
Update .gitignore rules.
WIP: update windows installer.
Transition to PHP 5.3.0 & components.
Remove windows installer files.

Development on the windows installer has been moved to a separate repository.
See https://github.com/Erebot/WindowsInstaller.