Add config option for SvnDriver to allow Symfony-like package structure in svn

This patch will add the *module-path* config option for svn-based repositories. This option allows placing package source code not in svn module root, thus making it possible to organize you library in the way Symfony2 of ZF2 frameworks do.

For example we have following repository structure:

/core-package-name/trunk/composer.json <-- complete library
/core-package-name/trunk/Company/Package/Class.php
/core-package-name/trunk/Company/Package/composer.json <-- single package

Or something like this:

/some-module/trunk/some-historical-path/Company/Package/Class.php
/some-module/trunk/some-historical-path/composer.json

In both cases Satis will fail to build packages.json (in first case for Package) because it will not be able to find composer.json.

The *module-path* is path from trunk/branches/tags to composer.json