Breadcrumb logging support

## Interface

```java
Bugsnag.leaveBreadcrumb(String whatHappened);
Bugsnag.setMaxBreadcrumbs(int max);
Bugsnag.clearBreadcrumbs();
```

## Rules

- Circular buffer of log messages
    - Default 20 messages
    - Configurable with `setNumBreadcrumbs`
- Truncate super-long breadcrumb messages at 140 characters

## Format

```
event.breadcrumbs = [
  ["2014-12-20T16:37:30Z", "App started"],
  ["2014-12-20T16:39:01Z", "User clicked login"]
]
```
:trollface: Is LinkedList not better for this?