Approval by email is not supported in the product but here is one way it could work. A workflow that is waiting for approval can be approved by calling the REST api
You could add a rule on your mail server to call this REST api whenenver an email from the manager/approver is recieved with a particular format. The api needs the jodId which can be obtained by enabling email notification from WFA. Alternatively you could also use the "Send email" command to mail the job id to the manager just before the approval point.
Here is a bit more about REST apis in WFA.
You can also see the apis avialable by pointing to the url http://<WFA-Server>/rest/docs/ or http://<WFA-Server>/rest/ . This might be one of the two depending on which version of WFA you are using.
One of the ways you can set up the Rule to invoke the REST api is to configure the rule to run a powershell script with the cmdlet Invoke-RestMethod. More information here https://technet.microsoft.com/en-us/library/hh849971.aspx