Option --log10 added to map2fig
New function hpix_valid_nside added
Many, many changes.

The following changes have been implemented in the source code:

1. Order conversion functions now closely follow the implementation found in the Healpix 3.00 C++ library. This required a large re-implementation of the internals of hpix_map_t.
2. Many bugs have been fixed

This commit makes the source code not usable nor (probably) completely compilable. This is going to be fixed very soon.
Improved documentation, test cases and examples.

Three major changes:
1. The documentation has been heavily revised and improved
2. A new directory "examples" has been added
3. Test cases are now implemented via the Check library (http://check.sourceforge.net/)
Merge conflict solved