SVN: use plugin api functions for lang and config

Replace calls to lang_get/config_get/config_set( 'plugin_SourceSVN_*' )
by plugin API calls (i.e. plugin_lang_get, plugin_config_get,
plugin_config_set)
Rename check_svn() to svn_check()

To be consistent with naming of other svn function calls
SVN check should use the command-line options

Using svn_call() instead of svn_binary() method, so that command
arguments defined at plugin level are used in svn_check().

Fixes #81
SVN: bump version