plugin: ftp: Add plugin service API.

Issue #128.
plugin: ftp: Add size info
plugin: ftp: Add plugin skeleton.

Based on QGroundContol QGCUASFileManager.h/cc.
Issue #128.
plugin: ftp: Fix CRC32 calculation.

Issue #128.
plugin: ftp: Implement list parsing

Issue #128.
plugin: ftp: Implement FTP:List method.

Issue #128.
plugin: ftp: Implement FTP:Open (read) and FTP:Close.

Issue #128.
plugin: ftp: Fix open error.

Issue #128.
plugin: ftp: Implement FTP:Read call.

Issue #128.
plugin: ftp: Implemnet reset call.

Sometimes kCmdReset can restore normal operation,
but it might be dangerous.
Issue #128.
service: mavftp: Initial import.

Issue #128.
Merge pull request #145 from vooon/ftp-plugin-128

FTP plugin