Add generating TAGS at other directory(#11)

If users call 'helm-gtags-update-tags' with 'C-u C-u' prefix,
users can generate global tags at other directory.
Introduce update interval for auto updating tags(#11)
Merge pull request #12 from syohex/pr11

Support #11 partially
Bump up version and update Changes
Implement history navigator(#14)
Merge pull request #15 from syohex/support-history

Implement history navigator(#14)
Update document

 - add explanation of helm-gtags-next-history and helm-gtags-previous-history.
 - remove old information
 - fix sample configuration based on package.el or el-get
Add message about GNU global version
Bump up version and update Changes
Support GTAGSLIBPATH(#13)

Use previous context stack if current path is under GTAGSLIBPATH,
and create new context stack if previous context stack in such case
Fix overwrite tag location problem
Merge pull request #16 from syohex/support_libpath

Support libpath(#13)
Apply shell quote for file name(#17)
Bump up version and update Changes
Update document
fix libpath issue(#13)
Merge pull request #18 from syohex/fix-libpath-support

fix libpath issue(#13)
Bump up version and update Changes