Implement new behavior for KVOing array w/ SSArrayDataSource

This gives SSArrayDataSource a distinct initializer that gives it the
KVO behavior of an array off of some object at a key path. All of the
previous tests are passing.
Use internal KVO minoring for plain 'ol SSArrayDataSource

Instead of having two paths for array manipulation and view updates
within SSArrayDataSource (between being handed an NSArray to manage and
KVOing a target keyPath), let's just always KVO, whether or not target
is an external object or self.
Simplify internal array proxy of items for SSArrayDataSource
Fix KVO observer leakage in SSArrayDataSource

This fixes the [observer leakage][n] without introducing a [possible
retain cycle][rc]. I'm not sure why `weak` doesn't work for us.

We still need handle the situation where `target` could get unexpectedly
get deallocated since it's no longer automatically nil'd out.

[n]: https://github.com/splinesoft/SSDataSources/pull/29#issuecomment-39612129
[rc]: https://github.com/splinesoft/SSDataSources/pull/29#issuecomment-39646222
Add a failing (EXC_BAD_ACCESS) test for target dealloc
Change `target` ownership to strong, fix -test_no_exception_if_target_goes_away

I'd still [cast a vote][c] that this will limit the functionality of
SSArrayDataSource's KVOing abilities, but I think it's a worthy
trade-off to prevent a crash.

[c]: https://github.com/splinesoft/SSDataSources/pull/29#issuecomment-39678236
Delegate array datasource items ownership to internal object

Takes the [design][d] suggested by @erikprice.

[d]: https://github.com/splinesoft/SSDataSources/pull/29#issuecomment-39655048
Add doc warning re: strong ownership of target
Merge branch 'array-kvo-deux' of github.com:andrewsardone/SSDataSources into andrewsardone-array-kvo-deux

Conflicts:
	SSDataSources/SSArrayDataSource.m
Bump to 0.6.0