Sam: add tests to the public API

**Added the following tests**:

* unclipped start and stop
* copy constructor tests
* copy assignment test
* tags of types char, int and double
* char_tag when tag is not a char
* string_tag when tag is not a string
* is_missing support for tags

major strides towards addressing issue #4
We'll have to add tests for type mismatches in the integer_tag/double_tag cases once we patch htslib to fix the ambiguity issues