Use Apple's provided base64 implementation.
Merge pull request #160 from square/kve/kve/RI-3784

[Master] Use Apple's provided base64 implementation.
Fix size conversion warnings.
Merge pull request #162 from square/alanf/fix-size-conversion-warnings

Fix size conversion warnings.

Reviewed by @firestone.
Merge branch 'master' of https://github.com/square/SocketRocket

Conflicts:
	SocketRocket.xcodeproj/project.pbxproj
	SocketRocket/SRWebSocket.m
	SocketRocket/base64.c