Better error handling + WriteFile support

- I added a little better error handling around GetDimensions.
- I added a WriteFile function as well as ImageType constants

On my machine, using NewPixFromFile and WriteFile to convert from
tif to png is 6x faster than ImageMagick and 2.5 times faster than
GraphicsMagick.
Added some documentation (and corrected old docs).
Merge branch 'master' of https://github.com/GeertJohan/go.leptonica
Merge pull request #2 from robertmeta/master

WriteFile support + a (very) little documentation