Match up documentation with that from std::collections::HashMap

[Arthamys]

This PR updates the documentation to match that of std::HashMap, along with simple examples.

The examples make use of flurry::epoch::pin(). I don't know if this is still desirable, or if the documentation should make use of HashMap::guard().

[jonhoo]

This is great! I'll take a look through once I get the chance.

Two high-level comments:

• We probably want to use @cuviper's new HashMapRef type in most of the examples, as it avoids the need to produce a guard manually. Just map.pin().get(key) for example. That should tidy up a lot of the example code I think.
• When there are comments in the documentation, you need to enclose them in /* */ so that they do not show up in the actual documentation :)

[cuviper]

Oh, the joys of concurrent data structures -- "look ma, no mut!"

[jonhoo]

I'm working on fixing the failing address sanitizer stuff in #59

[jonhoo]

Excellent, thank you so much!