Implement FileTest.size

- File.size delegate to FileTest.size
- File.size more faster by reading i-node value
Merge pull request #28 from ksss/file-size

Implement FileTest.size
Fix broken build for Visual C++
Merge pull request #29 from sdottaka/fix-for-msvc

Fix broken build for Visual C++
IO.sysopen does retry.  closes #26.