Adds a `fp_TXTRecord` category to NSNetService to allow for more natural access to TXT record data.

FPNetServiceTXTRecord.h

#import <Foundation/Foundation.h>

@interface FPNetServiceTXTRecord : NSObject
@property NSNetService *netService;
- (NSString *)objectForKeyedSubscript:(NSString *)key;
@end

@interface NSNetService (FPNetServiceAddtions)
- (FPNetServiceTXTRecord *)fp_TXTRecord;
@end

FPNetServiceTXTRecord.m

#import "FPNetServiceTXTRecord.h"
#import <objc/runtime.h>

@implementation FPNetServiceTXTRecord
- (NSString *)objectForKeyedSubscript:(NSString *)key {
    NSData *data = [NSNetService dictionaryFromTXTRecordData:self.netService.TXTRecordData][key];
    return (data != nil) ? [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding] : nil;
}
@end

@implementation NSNetService (FPNetServiceAddtions)
- (FPNetServiceTXTRecord *)fp_TXTRecord {
    FPNetServiceTXTRecord *txtRecord = objc_getAssociatedObject(self, @selector(fp_TXTRecord));
    if(!txtRecord) {
        txtRecord = FPNetServiceTXTRecord.new;
        txtRecord.netService = self;
        objc_setAssociatedObject(self, @selector(fp_TXTRecord), txtRecord, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    }
    return txtRecord;
}
@end