Add support for context

[mishudark]

The use of context is a must to have with Go v1.7.4+ and it should help to cancel long running process according the developer needs

[fabiante]

Where would you like to usage the context api?

Currently the places I can think of:

• EventStore.Load implementations - Because replaying lots of events is a potentially long running process.
• EventStore.Save and SafeSave implementations - Because event saving is also an iterative process that may take longer

There surely are some more.

I could add context support for the above methods as a starter if you'd like.

[mishudark]

Hello, yes those areas are interesting to add context support