Add a callback for filtering breadcrumbs

[fractalwrench]

This adds a callback which allows a developer to specify whether a breadcrumb should be captured or discarded. This also exposes the Breadcrumb class as part of our public API, and allows manipulation of the MetaData. Some duplication within the Breadcrumbs class has also been removed.

Addresses #221

[coveralls]

Pull Request Test Coverage Report for Build 1081

• 57 of 59 (96.61%) changed or added relevant lines in 5 files are covered.
• 7 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+0.3%) to 70.595%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
sdk/src/main/java/com/bugsnag/android/Bugsnag.java	0	2	0.0%

Files with Coverage Reduction	New Missed Lines	%
sdk/src/main/java/com/bugsnag/android/Breadcrumbs.java	2	93.33%
sdk/src/main/java/com/bugsnag/android/DeviceData.java	2	76.56%
sdk/src/main/java/com/bugsnag/android/Client.java	3	52.58%

Totals
Change from base Build 1079:	0.3%
Covered Lines:	1755
Relevant Lines:	2486

---

💛 - Coveralls

[bengourley]

is send the best name for it? If I understand correctly, this method decides if it will "save" not "send" the breadcrumb. I'd call it something like shouldSave(breadcrumb).

[fractalwrench]

Updated names in response to feedback from Duncan