[FIX] Using SYSTEM include_directories for Boost.
Merge pull request #559 from holtgrewe/feature/system-includes

[FIX] Using SYSTEM include_directories for Boost.
[FEATURE,DOC,TEST] Extending Gaps interface for counting.

Completing functions isGap(), countGaps(), isCharacter(), countCharacters()
for both Gaps and GapsIterator.

Includes documentation as well as update to tests.
Merge pull request #560 from holtgrewe/feature/gaps-api-ext

[FEATURE,DOC,TEST] Extending Gaps interface for counting.