Released gnet v1.2.0

#Features

• Support graceful shutdown with OS signals (Fixes #82)

#Enhancements

• Improve the creation processes of netpoller (Fixes #72) ccc6c25
• Reuse slice struct when resetting buffer 916c255
• Make a big allocation on ring-buffer at the first time 61963bd

#Docs

• Update Features section in READMEs a0d53f5
• Add a new article about gnet d500bf4
• Supplement the missing acknowledgment item b031357

#Misc

• Enable ReadN(int) to handle corner cases 2e89e25
• Leverage reviewdog as automated code review tool 5db42f9
• Use customized logger to log errors instead of std log package 3a69701
• Remove useless field of server 47817a0
• Unexport some errors 5287bd8