New symlink flag set by the compiler, new compile with symlinks checkbox in the setup (#190)

The flag consists of the new `data/.compilation-symlinks` file, which is included in the code base (but not available in the Combodo packages).

It will be used and generated if and only if : 
* we are on a dev env (`\utils::IsDevelopmentEnvironment`)
* the `symlink` PHP function exists

The flag is generated low level in the compiler (\MFCompiler::DoCompile)

In the setup, if the flag is present and all conditions are met then a new option will be displayed in the "Miscellaneous Parameters" wizard step. When unchecking/checking the flag will be updated accordingly by an ajax query.

All other compiler consumers (Designer / Hub connectors, Core Update, scripts using `RunTimeEnvironment` class) will : 
* if they provide a value for the symlink option, it will be used
* otherwise the flag will be used instead, if conditions are met

.gitignore

@@ -32,6 +32,8 @@ test/vendor/*
 !/data/.htaccess
 !/data/index.php
 !/data/web.config
+!/data/exclude.txt
+!/data/.compilation-symlinks
 
 # iTop extensions
 /extensions/**