* Enforce Range to take arithmetic type as template type.
Merge pull request #5 from vsfs/range_numeric_value

 * Enforce Range to take arithmetic type as template type.
 * Added Range::length()
 + Added more warnings
 + Added AUTHORS
 * Int2Type => IntToType
 * Update README.md about how to build API document
 * Fixed a stupid template naming bug
 * Use travis to test against g++ 4.8 and 4.7
 * Only upload g++-4.7's coverage report
Merge branch 'master' of github.com:vsfs/vobla into add_ch_ring

Conflicts:
	.travis.yml
* third modifcation according to code review
* updated comments.
* added definition of operator[].
* unified All Key to key_type.
* deleted the const keyword for key_type.