Use bufferevent_write_buffer in obfs2/3 for client data (Fixes #24)

The issue says to use evbuffer_peek()/evbuffer_add_buffer() but on
further investigation modifying data returned from evbuffer_peek() can
result in undefined behavior, and bufferevent_write_buffer() is the
correct call to use to send.

The changes should result in better performance, though I'm still not
totally thrilled about how the code currently pulls up the entire
buffer.