Import float.h to get hold of DBL_EPSILON

DBL_EPSILON is defined in float.h and I was getting a compile error that
it wasn't defined.
Test case for #31.
Apparently `TEST_AFTER_BUILD` doesn’t work anymore. Yay!
Added a destination specifier for iOS, so that xcodebuild doesn’t assume I want to run on a device that isn’t present.
Wrap the time-zone cache in an object that stores it in the thread dictionary. Should improve thread safety and thereby fix #12. (However, I have not tested it in a multi-threaded situation.)
Added test-case for Issue #36
Fixed Issue #36
Merge pull request #40 from iOSCowboy/Issue-36

Fix for Issue #36
Merge pull request #35 from kylef/float-header

Import float.h to get hold of DBL_EPSILON.
Added support for millisecond precision
Merge pull request #41 from rkeniger/master

Added support for millisecond precision
Merge branch 'master' of https://github.com/boredzo/iso-8601-date-formatter into barz-master