Fix no-return warning in FtpInitiateConnectionCommand

... and untangle code a bit while at it.
AppleTLSSession: Remove unused private member
Give hint on how to install sphinx-build

... and remove trailing parenthesis from warning messages
Add fast-path to percentEncode(string&)
Optimize htmlEscape implementation a bit
Fast-path for percentEncodeMini(string&)
Use AM subdir-objects

Doing so in AM_INIT_AUTOMAKE seems to be the most compatible way of
doing so.

Closes GH-120