Modify build() to deep copy the header into a VariantHeader
- Add one_time_build() for a more efficient move into a VariantHeader
- Add/refactor tests
Standardize move constructor and assignment
- delete copy constructor/assignment
- add move constructor/assignment test
Make VariantHeaderBuilder chainable
- Fixes #212
Merge pull request #253 from broadinstitute/jt_VariantHeaderBuilder_212

Chainable VariantHeaderBuilder