Bumped version to v2.6

Bumped version to v2.5

Bumped library version to v2.4

Bumped library version to v2.3

Merge pull request p-ranav#115 from HydrArgs/cpack

Some cleanup in CPack packaging

Merge pull request p-ranav#61 from zhihaoy/exit-on-help

Default --help to print help and exit

Minor test cases

Merge branch 'master' of github.com:p-ranav/argparse

Update README.md

Update README.md