Added explicit Enum type for PollResult.
Updated travis.yml.

- Use latest SDK version when possible.
- Run against iOS 8.1 SDK
- Added OS X to travis.
- Emit customizable environment variables
- Remove destination timeout
contains() for objc matches strings correctly.

Fixes #50.
Merge pull request #49 from jeffh/async-mainrunloop-stall-detection

Async matchers bailout on blocked main runloop.
Updated contain failure message if the objc matcher is using unsupported types.
Exit polling early if condition is already met
Merge pull request #54 from github/upstream-short-circuit-polling

Exit polling early if condition is already met
Reduce default toEventually() polling interval to 0.01s
Merge pull request #55 from github/upstream-poll-more-frequently

Reduce default toEventually() polling interval to 0.01s
Merge pull request #51 from jeffh/objc-contains-str

contains() for objc matches strings correctly.
Stripped underscores and made functions and methods explicitly internal
Matchers more uniformily handle nil actual values.

nils fails matchers in both to() and toNot() cases to prevent
accidental nil fallthroughs in Swift + Objective-C when writing tests.

This generally reduces the amount of expectations required in tests. Use
the beNil() matcher if comparing to nils is expected.

The following matchers currently pass when matching against nil:

- beNil()
- beFalsy()
- raiseException()

equals() is the only matcher that still currently checks its expected
value to be non-nil. This may change for other matchers that accept
expected arguments (eg - beGreaterThan, beGreaterThanOrEqualTo, etc.)
Fixed failing tests
Added tests for async and custom timers for objc