Changes for Mono use

Changed WebPFree to WebPSafeFree, because WebPFree is only available when compiling dll's for windows:

http://git.chromium.org/gitweb/?p=webm/libwebp.git;a=commit;h=45feb55decb31602190b1157f36b0813e18f6385

Also, changed DllImports to just look for "libwebp" instead of "libwebp.dll". We then use that as part of a dllmap to reroute to libwebp.so for linux.