is_missing renamed to missing

Renamed the is_missing functionality across the library to match the more STL-like missing() calls. Also renamed is_empty() to empty() in the sam_builder class. The two separate commits reflect both changes.

fixes #152
Also, as a general rule we should forbid whitespace changes like this in normal pull requests. They obscure the actual changes (here, just a single method is renamed, which took me a while to figure out), and they force the reviewer to wade through many extra lines manually looking for differences.