refactoring code
Fix docstrings
Code refactoring
Implement helm-gtags-display-browser

This is same command as `gtags-display-browser'.
Add `helm-gtags-find-pattern binding

This command can be used as 'gtags-find-with-grep'.
Implement helm-gtags-find-tag-other-window command
Merge pull request #50 from syohex/implement-gtags-el-commands

[WIP] implement gtags.el features