Add an option to pass in a Context to cancel Lua scripts

[QinanLi]

This PR adds a context.Context to the options struct which allows easy cancellation of running scripts.

As context was only introduced in Go 1.7 this would mean dropping support for previous versions of Go.

[coveralls]

Coverage increased (+0.006%) to 90.449% when pulling 4aec3cb on xencode:add-context into d0d5dd3 on yuin:master.

[yuin]

I'm afraid this would be difficult to accept.

1. GopherLua is already used in many projects, so silently dropping older versions is going to break these projects.
2. This functionality is implemented at debug.sethook in original Lua implementation. And debug.sethook is not implemented in GopherLua because it cause a little performance degradation and is not quite so common. I suspect this PR causes a performance degradation too.

[yuin]

@QinanLi , I've added a context support for GopherLua using "golang.org/x/net/context" .
golang.org/x/net/context has a fallback mechanism, so you can use same code for Go1.5, Go1.6 or Go1.7.
If you use Go1.7, golang.org/x/net/context uses builtin context package as its implementation.

And I completely separated VM execution path of the case using the context and the case not using the context. This means there is a no performance degradation when using GopherLua without a context.

[QinanLi]

@yuin that's great! It'll be good news for others with the same problem but I had a less restrictive use case that required less than the rework that you've implemented. All the same thanks a lot for your work.