Add -[OFMutableDictionary initWithCapacity:].

This was already implemented in OFDictionary_hashtable for internal
usage and is now publicly available.
A few small fixes for OS X/PPC.
Add -[OFMutableArray initWithCapacity:].
Rename -[OFStream pendingBytes].

It is now called -[numberOfBytesInReadBuffer].

Additionally, this commit renames OFStream's _cache to _readBuffer.
Rename OFXMLParser's _cache to _buffer.