Fix #99: Use svn_binary in svn_check

Fix #86, #94: Added SVN command argument configs ...

Extended the SVN configuration section to include a checkbox for
trusting SSL certificates, as well as a generic textbox to specifying
additional command arguments to the SVN binary.  Together, these two
options should allow repositories to be imported via HTTPS/SSL, as well
as additional flexibility for system admins.

Issue #97: SVN configuration uses new Source API

Fix #97: Refacter core SVN into shared codebase ...

Created base SVN plugin with the shared SVN code from WebSVN and SFSVN
plugins, and then modified the previous plugins to inherit from the new
base plugin.  This will end the previous need to constantly replicate
changes between the two SVN plugins.

Currently, the only way to modify shared SVN configuration is to have
the base SVN plugin installed, but this is a task for a different
commit.