Beta quality drop of Objective C Support.

- Add more to the ObjC dir readme.
- Merge the ExtensionField and ExtensionDescriptor to reduce overhead.
- Fix an initialization race.
- Clean up the Xcode schemes.
- Remove the class/enum filter.
- Remove some forced inline that were bloating things without proof of performance wins.
- Rename some internal types to avoid conflicts with the well know types protos.
- Drop the use of ApplyFunctions to the compiler/optimizer can do what it wants.
- Better document some possible future improvements.
- Add missing support for parsing repeated primitive fields in packed or unpacked forms.
- Improve -hash.
- Add *Count for repeated and map<> fields to avoid auto create when checking for them being set.

objectivec/GPBCodedInputStream.h

diff --git a/objectivec/GPBCodedInputStream.h b/objectivec/GPBCodedInputStream.h
index e9b27e2..b71ee87 100644
--- a/objectivec/GPBCodedInputStream.h
+++ b/objectivec/GPBCodedInputStream.h
@@ -60,7 +60,7 @@
 - (int64_t)readSInt64;
 - (BOOL)readBool;
 - (NSString *)readString;
-- (NSData *)readData;
+- (NSData *)readBytes;
 
 // Read an embedded message field value from the stream.
 - (void)readMessage:(GPBMessage *)message