open / save related bugs

[ngryman]

• [x] color mapped images do not support alpha: https://code.google.com/p/leptonica/issues/detail?id=82. This applies to 8-bit png and gif image formats.
• [x] alpha channel is set to 0 when the source file does not have alpha. This is problematic when saving to a format that supports alpha: it's full transparent. When not present, alpha channel should be set to 255.
• [x] pixels are in abgr order instead of rgba. It's possible to swap them with pixEndianByteSwap, but this has to be done twice: after read and before write.
• [x] quality for jpeg write seems to be ignored. The jpg is always of the same quality.

[ngryman]

This is mostly resolved or deferred.