Implement :force option in the app generator and the model generator

Closes #1712
Prepare log directory
`log` and `tmp` should contain .keep file, closes #1717
Cleanup a whitespace
Merge pull request #1719 from padrino/prepare-keepfile

Empty directories should contain .keep file, closes #1717
Merge pull request #1718 from padrino/implement-force-option

Implement :force option in the app generator and the model generator
do not remove external constants, fixes #1722
ignore JRuby ObjectSpace setting
add tests for #1723
Merge pull request #1723 from padrino/external-const

do not remove external constants, fixes #1722
use native Array#empty?
minor refactor of TagHelpers
do not apply default layout to templates with external engine, fixes #1727