CHE-4097: Refactor go agents package structure (eclipse-che#4344)

Also add comments and cleanup code.
Signed-off-by: Alexander Garagatyi <agaragatyi@codenvy.com>

Author: Alexander Garagatyi

agents/exec/.gitignore

@@ -0,0 +1,2 @@
+src/main/go/exec-agent/exec-agent
+src/main/go/exec-agent/logs

agents/exec/README.md

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+Summary
+---
+Golang based server for executing commands and streaming process output logs,
+also websocket-terminal.
+
+
+Requirements
+--
+- golang 1.6+
+
+
+Docs
+---
+- jsonrpc2.0 based [Webscoket API](docs/ws_api.md)
+- jsonrpc2.0 based [Events](docs/events.md)
+- [REST API](docs/rest_api.md)
+
+Development
+---
+
+##### Link the sources to standard go workspace
+
+```bash
+export CHE_PATH=~/code/che
+mkdir $GOPATH/src/github.com/eclipse/che/agents/go-agents/src/main -p
+ln -s $CHE_PATH/agents/go-agents/src/main/go $GOPATH/src/github.com/eclipse/che/agents/go-agents/src/main/go
+```
+
+That's it, `$GOPATH/src/github.com/eclipse/che/agents/go-agents` project is ready.
+
+##### Building linked project
+
+```bash
+cd $GOPATH/src/github.com/eclipse/che/agents/go-agents/src/main/go && go build ./...
+```
+
+#### Building exec agent executable
+
+```bash
+cd $GOPATH/src/github.com/eclipse/che/agents/go-agents/src/main/go/exec-agent && go build
+```
+
+##### Running linked project tests
+
+```bash
+cd $GOPATH/src/github.com/eclipse/che/agents/go-agents/src/main/go && go test ./...
+```
+
+##### Formatting linked project sources
+
+```bash
+cd $GOPATH/src/github.com/eclipse/che/agents/go-agents/src/main/go && go fmt ./...
+```