Fix a typo in the documentation.

This fix a typo in the README documentation.
Add usage examples for the ping command.

This adds usage examples for the ping command in the README file.
Merge branch 'master' of github.com:gawen947/wsn-tools
Use fadvise only when available.

The posix_fadvise is not always available on some platforms. This patch
enables the use of this function only on architectures that are known to
work.