Enable the use of context in restic

Set up a cancelble context in global options, hook it into the ctrl-C handler
for proper cancel propegation.

Bump up minimal requirement for Go to version 1.7 in documentation
and test-build files.

doc/Manual.md

@@ -27,7 +27,7 @@ $ pacaur -S restic-git
 
 # Building restic
 
-restic is written in the Go programming language and you need at least Go version 1.6.
+restic is written in the Go programming language and you need at least Go version 1.7.
 Building restic may also work with older versions of Go, but that's not supported.
 See the [Getting started](https://golang.org/doc/install) guide of the Go project for
 instructions how to install Go.