Add .gitignore file similar to Chromium's one.

Review URL: http://codereview.chromium.org/160638

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@2627 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

.gitignore

+*.a
+*.exe
+*.lib
+*.log
+*.map
+*.mk
+*.ncb
+*.pyc
+*.scons*
+*.suo
+*.user
+*.xcodeproj
+d8
+d8_g
+shell
+shell_g
+/obj/
+/tools/visual_studio/Debug
+/tools/visual_studio/Release
+/xcodebuild/
+TAGS