ios build: fixed toolchain for cmake 2.8.10
ios simulator build: set CMAKE_OSX_SYSROOT

When building for iphone simulator it is required to set the CMAKE_OSX_SYSROOT
to "iphonesimulator" otherwise command line builds (with cmake --build) will fail.
ios build: added option to set signing certificate

Code signing certificate can be set with with cmake option CODE_SIGN_IDENTITY.
Like -DCODE_SIGN_IDENTITY="signing identity"
Cleaned up and improved iOS openssl build script

* Added support for optional build directory (first argument)
* Automatically detect iOS/iPhoneSimulator SDKs (oldest SDK found is used)
ios ssl build: do sdk detection before download
ios ssl build: support for user specified sdks

To build against a specific (not auto detected) SDK version set
USER_OS_SDK and/or USER_SIM_SDK at the top of the script.
ios build: updated README.ios
Merge pull request #1534 from bmiklautz/iosbuild

IOS build improvements