Fix Issue #30: Reference Set Implementation

https://github.com/alobbs/libhpack/issues/30

Current implementation was not handling the reference set correctly, now it is fixed.

There's been a lot of changes, here's the summary:
- Created a Set implementation using bitmaps (bitmap_set.h, bitmap_set.c).
- Created QA tests for the Sets.
- Header Field is added an information Flag.
- Header Table is reimplemented to be more compact, to use the Sets and to properly use the Max. Size.
- Header Parser is added the Reference Set functionallity and has some issues fixed.
- Reverted QA tests to prevent regressions.
- Doxygen comments added for the bitmap_set, the header_field, the header_table and the header_parser.
- Defined SETTINGS_HEADER_TABLE_SIZE and HPACK_HEADER_ENTRY_OVERHEAD.

Conflicts:
	libhpack/macros.h
	test/header_table_test.c