switch to performSelector:; fix issue #17
document observer weak reference requirement; fix issue #16
increment podspec version
[pop] remove unecessary initial workaround

Summary:

Original benchmarking improperly accounted for external factors. Manually performing the callback is only marginally faster.
add optional observee weak reference support
use length modifier t for formatting NSUInteger without casting
Merge pull request #19 from cxa/master

use length modifier t for formatting NSUInteger without casting
Fix typo
Merge pull request #21 from zzz6519003/patch-1

Fix typo
Added convenience methods for observing multiple paths with same block/action/context
Merge branch 'danielvy-observe_multiple_paths'
update Travis CI to xctool
improve unit test robustness