svn export --ignore-externals if specified :externals => false
add subversion-refs-externals-repo to fixtures for test
add test for svn externals
[CHANGELOG] svn export --ignore-externals
Merge pull request #8 from banjun/svn-ignore-externals

svn export use --ignore-externals option if specified :externals => false
[HTTP] Fix path shell escaping during downloading and unpacking.