Optimized NMSFTP append

Added appendStream:toFileAtPath:
Force to use SCP in non-blocking mode [issue #42]
Merge pull request #41 from touta/feature

Add method to get supported authentication methods
Code cleanup
Fix bug: Work with multi-byte char code
NSString stringWithUTF8String:  or  appendFormat:@"%s" read cString from head and stop on \0; despite multi-byte can contain \0.
Using method with length-specify like initWithBytes:length:encoding: works fine.
Add fclose to close file
Remove double-processing of closeChannel