Bug fix and minor recommendations

[NGraphApp] Fix memleak after destruction of inst (forgot to delete m_app)
[Refactoring] Remove all std strings, use Qt styled debug instead
[Refactoring] Don't compare with empty string (especially std's). Use QString::isEmpty() instead.
[Build] Removed unnecessary include of QtMath which prevents build with Qt 5.0.2
[Git] Added .gitignore file

Signed-off-by: Alexey Knyshev <alexey.knyshev@gmail.com>